Chapter VIII: PHYSIOLOGY, MEDICINE, ETC.--Histological Methods.--Section (2)

The annexed illustration shows the essential parts of Hahlo and Liebreich's improvement, the loom being now at work. The handrail, shuttle race, and starting handle can be at once recognized, and the shuttle guard will be seen in its proper position, which position it rigidly retains as long as the loom is working, but on a stoppage the rod swings back close underneath the handrail, and quite clear of the reed. The mode in which this is accomplished we will endeavor to make clear. The guard is connected to the starting lever by the arrangement shown, consisting of a stud on the handle, on which, with the movement of the slay, lever, a, slides. This lever, by means of another lever and a link, is attached to the shuttle guard by the crank, b, which, by means of the set screw in the boss, permits the shuttle guard to be adjusted in the most convenient place. It will be observed that whenever the loom stops working, whether it is stopped by hand or automatically, the hand lever has to be moved, and this movement is communicated to the shuttle guard by the mechanism just described, placing the guard rod beneath the hand rail, and leaving the whole of the shuttle race free and unencumbered. The act of starting the loom brings the guard again to the working position without any extra act having to be performed by the weaver. The action is entirely automatic, and the weaver has not anything to do that she has not to do with the present unguarded looms. The arrangement appeared to ourselves to be a very efficient one, and it has the merit that the length of the guard can be made greater than the width of the cloth, a further advantage that will be recognized by practical weavers.

* * * * *

RULER AND TRIANGLE FOR HATCHING.

The instrument shown in the cut is the invention of Mr. Maginnis, and is designed for producing equidistant hatchings. It consists of a short ruler, A, and a triangle, B, supposed to be one of 45°, but which may be of any angle. The triangle carries two stops, c c, while the ruler is provided with a conical piece, D, which is slotted, and is held by a screw. The play that occurs between this conical slide and the stops varies according to the position of the former.

The apparatus operates as follows: In the figure, the stop to the right being in contact with the piece, D, a line is drawn along the right side of the triangle. Then the ruler is made to slide along the triangle until D touches the other stop, and then the triangle is slid along the ruler until the stop to the right touches D again. In this position another line is drawn, and so on. The position of the piece, D, between the stops is regulated according to the fineness of the hatching to be done.--_Chronique Industrielle_.

* * * * *

THE DISTILLATION OF SEA WATER.

The supplying of the troops at Suakim and in the Soudan with water is one of the most important items in the whole conduct of the Egyptian war. Even in cold or temperate latitudes fresh water is a first necessity for animal life; much more is this the case in the desert; and the wells in the country forming the scene of our military operations form in themselves valuable strategical points. Their supply, however, has to be supplemented, and to do so artificial means and the aid of the engineer have to be enlisted into this service.

Many of our readers see notices from time to time in the newspapers about this or that ship being employed, or at least her steam fittings, in distilling water for the use of the troops; and although most of, if not all, our readers are engineers, still it is no disparagement to some of them to assume that they are more or less unfamiliar with sea water distillation on the scale on which the process is now being carried on at Suakim; and as the subject is of general interest, we give a short description of the process.

In a general sense, fresh water is obtained from sea water by simply generating steam from the sea water, passing the said steam through a surface condenser, and filtering the resulting water. The obtaining of fresh water in this way has been in practice on board sea-going ships for many years. It is supposed by some authorities on this subject that the first time fresh water was thus obtained at sea was by an old captain of a brig which ran short of water, and he cut up some pewter dishes into strips, which he bent and soldered into a pipe. He, with the carpenter's aid, fitted a wooden lid in one of the cooking boilers, and fixed one end of his pipe in it. He next sawed a water cask in half, bored a hole in the bottom of one half, and took his pipe through it, filling the space round the pipe with sea water. Thus he extemporized a worm and still or condenser. The distilled water, however, was scarcely drinkable. Not to be beaten, however, the captain got some pieces of charred wood which he put in the water, which so far improved it as to render it at all events fit to sustain life, and our skipper brought his brig and her screw safely to port. What suggested the use of charcoal to his mind history does not tell. For many years past scarce any sea-going vessel leaves port that is not fitted with a properly constructed distiller; and one conspicuous advantage attending this practice is that each ship thus fitted to the satisfaction of the Board of Trade inspector is allowed to sail with only half the quantity of fresh water on board which she should have if not provided with a distiller. The distiller and filter occupy very much less space than that which would be occupied by the casks or tanks of water otherwise required to be carried.

Coming now a little to detail, sea water distillers are usually fitted in connection with the winch and its boiler, which latter supplies the steam both for distillation and to drive the engine working its circulating pump. Smaller distillers are worked without a pump, the cooling water merely passing through by gravitation. These smaller affairs again are of two kinds, the one being mounted at one end of the cooking hearth, as in outline sketch, which shows a two oven hearth with distiller at one end. A is the supply pipe to admit air to aerate the water; B is the cock where fresh water is drawn off; C is a pipe conveying cooling water to the condenser E, placed on three little feet on top of the boiler, F, whose steam rises up a central pipe to the dome top, where it expands out and returns downward through a number of tubes about 1 in. diameter, in which it is condensed, collected in a bottom chamber, and drawn off through the cock, B. A distiller of this size would make about thirty gallons of fresh water per day. Very frequently a distiller, such as is shown in the sketch, is mounted separately, and placed near the winch or donkey boiler, which supplies it with steam, the lower part, F, being then used as a filter. The diameter of E is from 15 in. to 18 in., the outer casing being either iron or copper. Another form of distiller is one like the above, but larger, and having a small donkey engine and circulating pump attached thereto. As a rule these distillers are vertical, but larger apparatus are arranged horizontally. To give our readers some general idea of size, weight, and produce of water, we may say that a plain cylindrical distiller, mounted on a square filter case, measuring 3 ft. 9 in. high, weighing 4½ cwt., will distill twelve gallons per hour. A larger size, measuring 6 ft. 2 in. high, and weighing about 23 cwt., will give 85 gallons; while a still larger one, measuring 7 ft. high and weighing 32 cwt., yields 150 gallons. These have no pumps. When an engine and pump are fitted, the weight is increased from about 80 per cent. in the smaller to 50 per cent. in the larger sizes. An immense advantage attends the use of those distillers that are combined with a winch boiler. Of course, the chief use of the winch is while in dock; some use is made of it at sea to do heavy pulling and hauling, to wash decks, and in case of emergency the circulating pump is used as a fire engine. Were it not, however, for the distiller, the winch boiler would simply be idle lumber at sea. The distiller, however, finds useful employment for it, and has also this excellent effect, that as steam is pretty constantly kept up for the distiller, in the evil event of a fire the boiler is ready to work at once. In horizontal types of distiller an engine and pump are mounted on a cast iron casing as a bed, and in this casing is placed a number of tubes through which the steam passes to be condensed, the whole being simply a surface condenser with engine and pump above. Another type is that of a small single-flued horizontal boiler with combustion chamber and twenty or thirty return tubes--in fact, the present high-pressure marine boiler on a small scale. A boiler of this sort, measuring 4 ft. to 5 ft. long, 3 ft. 9 in. to 4 ft. 6 in. diameter, would have a horizontal donkey engine on a bed at its side, and at the end of the engine a vertical cylindrical condenser.

Few have done more, perhaps none so much, as Dr. Normandy to make sea water distillation not only a success as a source of water supply, but also to supply it at a minimum cost for fuel. He by a peculiar arrangement of pipes embodied something of the regenerative system in his apparatus, using the heat taken from one lot of steam to generate more, and again the heat from this he used over again. The defect of his older arrangements was undue complexity and consequent trouble to keep in order.

As can be well imagined, the distillers in use at Suakim are on a much more colossal scale, and owing to the now almost universal use of surface condensers in ocean steamers, no great difficulty ought to attend the adaptation of the boilers and condensers of one of our transports. One of these full-powered steamers will indicate, say, 5,000 horse-power, and assuming her engines to use 25 lb. of steam per indicated horse-power, or 2¼ gallons, she could distill some 12,000 gallons of water per hour. As no appreciable pressure of steam need be maintained, the boilers would suffer little from deposit, especially if regularly blown out. Hard firing need not be resorted to; indeed, it would be injudicious, as, of course, priming must be carefully guarded against. Of course, the salt water distilled will affect the working, not exactly of the distillers, but of the boilers. If the water in the harbor, as is not improbable, is muddy, some method of filtering it before pumping it into the boilers ought, if at all practicable, to be resorted to, for the twofold reason of preserving the boiler plates from muddy deposit, and also to prevent priming, which would certainly ensue from the use of muddy water. No doubt the medical staff take care that the distilled water is alike thoroughly aerated and efficiently filtered. The most successful method of aerating is, we believe, to cause the current of steam as it enters the condenser to suck in air by induced current along with it. The filtering ought not to present any difficulty, as at all events sand enough can be had. Charcoal, however, is another affair, and all distilled water ought to be brought into contact with this substance.

Simple, however, as such an arrangement as this appears to be, practical difficulties, which it is _said_ are insurmountable, stand in the way of its adoption, and the distilled water produced for Egypt is made in special apparatus, and various forms of condenser are employed, made under various patents. The principle involved is, however, in all cases the same. Steam is generated in one of the ships' boilers, and condensed, filtered, and aerated in a special apparatus. The great objection to the use of the ordinary surface condenser is that the main engines would, in the majority of cases, have to be kept going, in order to pump the distilled water out of the condenser, and to supply circulating water. But it is easy to see that if engineers thought proper, this difficulty could be readily got over. Separate circulating pumps, usually centrifugal, are now freely used, and the addition of a special pump for lifting the condensed water presents no difficulty whatever. While the main engines are running, the withdrawal of much condensed water would no doubt risk the safety of the boiler; but in the case of so-called "distilling" ships, there need be no trouble incurred on this score.--_The Engineer_.

* * * * *

AIDS TO CORRECT EXPOSURE ON PHOTOGRAPHIC PLATES.

[Footnote: We take from the Br. Jour. of Photo. the following interesting paper read by W. Goodwin before the Glasgow and West of Scotland Amateur Association.]

With good plates, and intelligent development, a practiced photographer may within certain limits correct the effects of an over or under exposure; but you have all, doubtless, found out that there is a correct exposure, and that you cannot trespass very far on either side of it without sacrificing something in the resulting negative.

MR. W.K. BURTON'S TABLE OF COMPARATIVE EXPOSURES ------------+--------------------+-------------------+------------------------- | | Badly lighted| Portraits in bright | | interiors,| diffused light Aperture | +------------+ up | out of doors. calculated | Landscape with | Fairly | to | / on the | heavy foliage in | lighted | | / Portraits in standard | foreground. | interiors | | / studio light system +------+-------+ +------+ | | | / of the | Sea |Open | | Under| | | | / Portraits Photographic| and |land- | |trees,| | | | | in ordinary Society. | sky. | scape.| |up to | | | | | room. ------------+------+-------+-----+------+-----+------+------+-----+------ | sec | sec | sec | m s | m s| h m | sec | m s| m s No. 1, | 1/160| 1/50 | 1/8 | 0 10 | 0 10| 0 2 | 1/6 | 0 1| 0 4 or f/4 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------ No. 2, | 1/80 | 1/25 | 1/4 | 0 20 | 0 20| 0 4 | 1/3 | 0 2| 0 8 or f/5.657 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------ No. 4, | 1/40 | 1/12 | 1/2 | 0 40 | 0 40| 0 8 | 2/3 | 0 4| 0 16 or f/8 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------ No. 8, | 1/20 | 1/6 | 1 | 1 20 | 1 20| 0 16 | 1-1/3| 0 8| 0 32 or f/11.314 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------ No. 16, | 1/10 | 1/3 | 2 | 2 40 | 2 40| 0 32 | 2-2/3| 0 16| 1 4 or f/16 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------ No. 32, | 1/5 | 2/3 | 4 | 5 20 | 5 20| 1 4 | 5-1/3| 0 32| 2 8 or f/22.627 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------ No. 64, | 2/5 | 1-1/3 | 8 |10 40 |10 40| 2 8 |10-1/2| 1 4| 4 15 or f/32 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------ No. 128, | 4/5 | 2-2/3 | 16 |21 0 |21 0| 4 15 | 21 | 2 8| 8 30 or f/45.255 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------ No. 256, |1-1/2 | 5-1/2 | 32 |42 0 |42 0| 8 30 | 42 | 4 15|17 0 or f/64 | | | | | | | | | ------------+------+-------+-----+------+-----+------+------+-----+------

The estimation of this correct exposure is probably the greatest difficulty in photography, and it is particularly discouraging to find plate after plate useless because the guess has been wide of the mark. There are some here to-night who have spoiled so many plates that at last they are prepared by experience for almost any contingency, and to those I nave very little to say; but there are also many who are still in their troubles, and I propose to tell them how the amount of guesswork required may be reduced to a minimum.

The factors which govern exposure are: the subject of the picture, the lens and its aperture, the rapidity of the plate, and last, but not by any means least, the quality of the light by which the work is to be done.

Let us consider each of these separately, and see if we cannot reduce any of them to rule. In this respect the subject will be found somewhat intractable. Scarcely two subjects will be found to send exactly the same amount of light through the lens. However, a broad classification may be made, and this has been done by Mr. Burton in his Table of Comparative Exposures. A glance at this table will show how greatly the character of the view may influence the time of exposure. Thus, with full aperture of a rapid symmetrical, the exposure for open landscape is given as one-twelfth of a second; when heavy foliage appears in the foreground, half a second will be required; while, under trees, as much as forty seconds may be needed.

The first aid I have to suggest is the use of such a table as Mr. Burton's. Before we do anything more in this direction, we must consider the influence of the lens and its diaphragms. In theory the single landscape lens is more rapid than the doublet of equal aperture, but the difference is so little that it may be disregarded in practice, and my remarks will apply to both.

The rapidity of a lens depends mainly on its aperture and its focal length. Thus a lens of twelve inches focus will require four times the exposure of a six inch, with an equal sized diaphragm, and a quarter inch diaphragm will require four times the exposure of a half inch when used in the same lens.

The Photographic Society of Great Britain have recommended that the diaphragms of all lenses should bear such relation to the focal length that each should require exactly double the exposure of the next smaller. Now, if we turn again to Mr. Burton's table, we shall find that it is constructed on this principle, and that each stop is numbered so as to show its exposure. Obviously, the most sensible thing would be to get a set of stops made to correspond with this arrangement, but we will see how we can construct a table for stops of any size.

First, if possible, find the equivalent focus of your lens. If it is made by a known maker, you will find it in his price list, and if not, you may calculate it for yourself by the rules given in the various text books, provided you have a camera of pretty long focus. However, it will be near enough for our purpose if you get a sharp image of the sun on a piece of paper, and while you hold lens and paper, get some one to measure the distance from the paper to the diaphragm aperture, or, in the case of a single lens, to the center of the lens. Note down this focal length, and proceed to measure your diaphragms in sixteenths of an inch.

Then, with pen and paper, proceed to divide the diameter of each stop into the focus, and state the result as a fraction of the focus, thus f/8. For example, a Ross half plate rapid symmetrical has a focal length of 7½ in.; for convenience reduce this to sixteenths=120. A diaphragm measuring seven sixteenths will give the fraction f/17. Now let us see if any of these stops correspond with Mr. Burton's. The first two in his table will only be found in portrait lenses, but we shall probably find one to correspond with the third, if we are using a doublet lens; with a single lens we won't find any so large. Having picked out those that correspond, and filled in the exposure for them, we have now to deal with the odd sizes. Here is one, f/27, which is just half way between No. 16 and No. 32, but a moment's thought will show that as the exposure increases as the square of the diameter, it won't do to take the exposure half way between the two.

We have another factor to consider now: that is, the rapidity of the plate. If you use plates by a maker who has a name to sustain, you may be pretty confident that they are of fairly uniform rapidity, so after you have got into the way of working any particular brand, the best thing you can do is to stick to it. The exposures in our table are for plates of medium rapidity in good spring light. In my own experience I find that they just suit "thirty times" plates, or fifteen on the sensitometer; but then I like a full exposure with slow development, and I know that others find these exposures just right for "twenty times" plates developed in the usual way. The most rapid plates in the market will not be overdone with half the given exposures. It must always be borne in mind that an error of a fraction of a second in either direction may be corrected in development, and it is impossible to make a very serious error if you refer to the table.

We come now to the light. If you depend on the eye entirely in judging the quality of the light, it will sometimes play you tricks. The rays which are most active on the plates are those which have the least effect on the eye. We can, however, by chemical means arrive at an exact estimate of the active power, and for this purpose an actinometer is used. This is simply an arrangement whereby a piece of sensitized paper is exposed and allowed to darken to a standard tint, and by the time it takes to reach that tint the value of the light is judged. Capt. Abney has, however, pointed out that ordinary sensitized paper is not suitable for bromide plates, since there are conditions of light in which the plates will be fairly rapid while the paper will be very slow. He gives a formula for a bromide paper, which is treated with tannin in order to absorb the bromine set free during exposure, otherwise the darkening would be very slight. I used this paper for a while, but found it rather slow. The tannin also turned brown on keeping for a week or so. I then made some more, substituting for tannin potassium _nitrite_ (not nitrate), which is colorless. This was an improvement, but still it was just slow enough.

However, noticing in Capt. Abney's article the statement that the bromide of silver should be as nearly as possible in the same state in the paper as in the plate, I thought "Why not Morgan's paper?" This, of course, is just bromide emulsion on paper, and if, as I suspect from its color, it contains a trace of iodide, why, so do most commercial plates. A sheet of this paper cut into strips, soaked for ten minutes in a fifteen-grain solution of potassium nitrite, and dried, gives a sensitive paper which darkens with great rapidity to a good deep tint, and keeps indefinitely. Here is some prepared last summer, which is still quite good. To use this paper make a little box so that a little roll of it can be stored in one end, and drawn forward as required beneath a piece of glass.

Bearing in mind that your table of exposures is calculated for the best spring light, go to the country some bright day next month with note-book, actinometer, and the necessary appliances for exposing a few plates. Select, say, an open landscape, and use your smallest stop. When all ready to expose, get out your actinometer and expose it to the reflected light of the sky for ten seconds (if the sun is shining, turn your back to it, and keep the actinometer in your own shadow); then put it in your pocket, expose a plate according to your table, and in case the light or plate should not be just in accordance with the conditions under which the table was prepared, expose other two plates, one a little less and one a little more than that first exposed. Then note down everything you have done--kind of view, stop, speed of plate, exposure of each plate, and length of exposure of actinometer.

When you get home, the first thing to do is to get hold of a paint box and paint the underside of the glass of your actinometer to match the darkened paper. Do this by gas light. Then scrape away a little of the paint, so as to let a strip of the paper be seen below it. After this develop your three plates with a developer of normal strength, and see which is best. If you have chosen a really bright spring day, and are using plates of medium rapidity, you will most likely find that exposed according to the table just about right.

Now let us see how we can use these aids in our field work. We have ascertained the correct exposure with a given stop on one class of view, with light of a given quality, but now suppose all these conditions altered. Let the view have heavy foliage coming close up to the camera, the stop be a size larger than that used in our first experiment, and the day rather dull. The table tells us what the exposure would be with this stop on this view, on a bright day; and if the actinometer take twenty seconds to reach the painted tint, then we must double the exposure given in the table.

You may sometimes find that the actinometer indicates a very different exposure from what the eye would lead you to expect. For instance, one day last September I went to Bothwell Castle, to get a picture I knew of in the grounds. It was one of those strange yellow days we had then, and the sun, though shining with all his might, was apparently shining through orange glass. The actinometer indicated an exposure of thirty seconds where in good light one would be right. I was rather incredulous. Thirty seconds in broad sunshine! However, I gave this exposure, but for my own satisfaction I gave another plate fifteen seconds only.

On developing, the latter was hopelessly underexposed while that having thirty seconds gave a negative which furnished one of my exhibition pictures.

I have shown you how to reduce the quality of the light to a certainty, also how to reduce to rule the exposure with different lenses and stops on certain classes of subjects, and it remains with you only to guess correctly to what class the view you wish to take belongs; I can assure you from my own experience that there is enough uncertainty about that point to prevent good negatives ever becoming monotonous.

The only aid I can suggest in this case is the continual use of a note-book. Note every plate you expose, and when you have a failure be careful to record the fact, and you will gradually find these accumulated notes becoming a great help in cases of doubt. One hint I can give to beginners is that a great number of the pictures to be met with in this part of the country are intermediate between "Open Landscape" and "Landscape with heavy foliage in foreground;" and it is scarcely needful to say that if you are in doubt, let the exposure be rather too much than too little; you _may_ make a negative of an overexposed plate, but never of an underexposed one.

* * * * *

ISOCHROMATIC PHOTOGRAPHY.

[Footnote: Read at the stated meeting of the Franklin Institute, March 18, 1885.]

By FRED. E. IVES.

It is well known that the ordinary photographic processes do not reproduce colors in the true proportion of their brightness. Violet and blue photograph too light; green, yellow, orange and red, too dark. For a long time it was believed to be impossible to remedy this defect; and even when it became known that bromide of silver could be made more sensitive to yellow and red by staining it with certain dyes, the subject received very little attention, because it was also known that the increase of sensitiveness was too slight to be of practical value in commercial photography.

Dr. H.W. Vogel, who was one of the first, though not the first, to devote attention to this subject, announced, in 1873, that he had succeeded in making a yellow object photograph lighter than a blue or violet one, by using a silver-bromide plate stained with coraline, and exposed through a yellow glass. The plate showed no increased sensitiveness to red, and the experiment, although of considerable scientific interest, did not indicate a practically useful process.

In the spring of 1878 I became interested in this subject, and tried to discover a method of producing plates which should be sensitive to all colors, and capable of reproducing them in the true proportion of their brightness. I commenced by trying nearly all the color sensitizers which had already been suggested, in order to learn which was the best, and then, if possible, _why_ it was the best, as a guide to further research. Chlorophyl was the only thing I tried which was sufficiently sensitive to red to offer any encouragement in that direction; but the solution which I obtained was weak and unstable, and far from being a satisfactory color sensitizer. Hoping to obtain a better solution with which to continue my experiments, I made extracts from many kinds of leaves, and found that a solution from blue myrtle leaves looked better and kept better than any other, and when it was applied to the silver-bromide plates they became remarkably sensitive, not only to all shades of red, but also to orange, yellow, and green. By placing in front of the lens a color-screen consisting of a small glass tank containing a weak solution of bichromate of potash, to cut off part of the blue and violet light, I obtained, with these chlorophyl plates, the first photographs in which all colors were reproduced in the true proportions of their brightness. But my chief desire at that time was to realize a method of producing from any object in colors a set of three negatives, in one of which the shadows should represent the blue of the original, in another the yellow, and in another the red, in such a manner that transparent pigment prints from these negatives--blue, yellow, and red--would, when superimposed on a white surface, represent not only the lights and shadows, but also the colors of the object. This had already been attempted by others, who failed because their plates were not sufficiently sensitive to red and yellow.

Having succeeded perfectly in my undertakings, I published my discovery in 1879,[1] explaining how to prepare and use the chlorophyl plates, in connection with the yellow screen, for the purpose of securing correct photographs of colored objects.[2]

[Footnote 1: _Philadelphia Photographer_, December, 1879, p. 365.]

[Footnote 2: I intended this publication to be a very full and explicit one, and it was sufficiently so to be perfectly understood by most who saw it; but some may think I did not sufficiently emphasize the importance of using the particular kind of chlorophyl which I mentioned. In a brief communication to the editor of the _Photo. News_, in 1883, I described some experiments with eosine as a color sensitizer, and then called attention to the superiority of blue-myrtle chlorophyl for this purpose, stating that I had not been able to secure such results with any other kind of chlorophyl, and that a fresh solution from fresh leaves must be used to secure the greatest possible degree of sensitiveness. See _Photo. News_, Nov. 1883, p. 747.]

So far as I know, nobody tried the process. Nearly five years later Dr. Vogel announced that, after eleven years of investigation, he had at last realized a successful process of this character, and that this new process of his was the "solution of a problem that had long been encompassed with difficulty." This publication attracted a great deal of attention, and gave me occasion to again call attention to my process,[3] and point out that it was not only the first practical solution of this problem, but the only truly isochromatic process ever discovered. Dr. Vogel's new process was not only no better in any respect, but the plates were insensitive to scarlet and ruby-red, and therefore would not photograph all colors in the true proportion of their brightness.

[Footnote 3: _Photo. News_, London, September 5, 1884, p. 566, and _Year Book of Photography_ for 1885, p. 111.]

My method consists in treating ordinary collodio-bromide emulsion plates with blue myrtle chlorophyl solution, exposing them through the yellow screen, and then developing them in the usual manner. The emulsion which I have employed is made with an excess of nitrate of silver, which is afterward neutralized by the addition of chloride of cobalt; it is known as Newton's emulsion. I now prepare the chlorophyl from fresh blue myrtle leaves, by cutting them up fine, covering with pure alcohol, and heating moderately hot; the leaves are left in the solution, and some zinc powder is added, which helps to keep the chlorophyl from spoiling. I have a bottle of this solution which was prepared about six months ago, and now appears to be as good as when first made.[4] A glass plate is flowed with the emulsion, and as soon as it has set, the chlorophyl solution is applied for a few seconds, after which the plate is washed in pure water until smooth, when it is ready for exposure.

[Footnote 4: I originally recommended chlorophyl extracted from dried leaves, because I had not yet learned how to preserve the solution for more than a few weeks; and at some seasons it would be difficult, if not impossible, to obtain fresh leaves. The tea organifier which I recommended is also a color sensitizer, and when it is used in connection with the chlorophyl from dried leaves the plates are as sensitive to red as can be safely prepared and developed in the light of an ordinary photographic "dark-room." Plates prepared with chlorophyl from fresh leaves do not require treatment with the tea organifier to secure this degree of sensitiveness. Recently I have used the tea organifier and some other sensitizers, in connection with the solution from _fresh_ myrtle-leaves, and in this way have produced plates having such an exalted color sensitiveness as to be unmanageable in ordinary "dark-room" light. Possibly, such plates might be prepared and developed in total darkness, by the aid of suitable mechanical contrivances, but I am not sure that they would work clear even then, because they appear to be sensitive to heat as well as to light.]

My color-screen consists of a small plate-glass tank, having a space of 3/16 of of an inch between the glass, filled with a solution of bichromate of potash about one grain strong. I place the tank in front of the lens, in contact with the lens-mount. The advantage of this tank and solution is that it can be more easily obtained than yellow plate glass, and the color can be adjusted to meet any requirement.

The plates require about three times as much exposure through the yellow screen as without it, and may be developed with the ordinary alkaline pyro-developer.

In order to illustrate the value of this process, I made two photographs of a highly-colored chromo-lithograph, representing a lady with a bright scarlet hat and purple feather, a yellow-brown cape and a dark-blue dress. One, by the ordinary process, represents the blue as lighter than the yellow-brown, the bright scarlet hat as black, and the purple feather as nearly white. The other, by the chlorophyl process, reproduces all colors in nearly the true proportion of their brightness, but with a slight exaggeration of contrast produced purposely by using a too-strong color solution in the small tank.

I also made two landscape photographs, one by the ordinary process, and the other by the chlorophyl process, exposing them simultaneously. In the ordinary photograph, distant hills are lost through overexposure, yet the foreground seems underexposed, and yellow straw-stacks and bright autumn leaves appear black. In the chlorophyl photograph, the distant hills are not overexposed, nor is the foreground underexposed; the yellow straw-stacks appear nearly white, and bright autumn leaves contrast strongly with the dark green about them.

To test the relative color-sensitiveness of plain emulsion plates, plates stained with eosine, and plates stained with the blue-myrtle chlorophyl, I exposed one of each kind through the same yellow screen, giving each five minutes exposure, on the same piece of copy, which was the chromo-lithograph already described. The plain emulsion plate showed only the high lights of the picture, after prolonged development. The eosine plate was underexposed, but brought up everything fairly well except the scarlet hat, which came up like black. The chlorophyl plate was overexposed, brought out all colors better than the eosine plate, and gave full value to the bright scarlet of the hat, the detail in which was beautifully rendered.

Dr. Vogel advanced the theory that silver-bromide is insensitive to yellow and red, because it reflects or transmits those colors; and that it becomes sensitive when stained, because of the optical properties of the dyes. He afterward admitted that only such dyes as are capable of entering into chemical combination with the silver-bromide proved capable of increasing its sensitiveness to color, but he held to the theory that the optical properties of the compound were the cause of its color-sensitiveness.

I have shown that the color-sensitiveness can be produced by treatment with an organic compound which has none of the optical properties characteristic of dyes; and that chlorophyl, which absorbs only red light, greatly increases the sensitiveness also to yellow and green. There is, therefore, good reason to doubt if the color-sensitiveness is ever due to the optical properties of the dye or combination.

Attempts have been made to produce isochromatic gelatine dry plates which, while many times more sensitive to white light than my chlorophyl plates, shall also show the same relative color-sensitiveness. Such plates would be very valuable but for one fact: it would be necessary to prepare and develop them in almost total darkness. Gelatine bromide dry plates extremely sensitive to yellow, but _comparatively insensitive to red_, might be used to advantage in portrait and instantaneous photography, because they could be safely prepared and developed in red light; but when truly isochromatic photographs are required, the time of exposure must be regulated to suit the degree of sensitiveness to red, which cannot safely be made greater than I have realized with my chlorophyl process.

* * * * *

DISTORTION FROM EXPANSION OF THE PAPER IN PHOTOGRAPHY.

The effect of the unequal expansion of paper, when wetted, in causing distortion of the photographic image impressed upon it, has, in the case of ordinary photographs upon albumenized paper, been well recognized; but the extent to which such distortion may exist under different treatment is worthy of some special consideration, particularly with reference to the method of printing upon gelatinized paper, which has been thought by some likely to supersede the method now usually employed with albumenized paper.

When a print upon the ordinary photographic (albumen) paper is wetted, the fiber expands more in one direction than in the other, so that the print becomes unequally enlarged, very slightly in one and much more so in the other way of the paper. When the paper is dried without any strain being put upon it, the fibers regain very nearly their original dimensions and position, so that the distortion which has existed in the wet condition nearly disappears.

If the photograph is cemented, while in the expanded condition, upon a rigid surface, the distortion then existing is fixed, and rendered permanent. Such a cementation or method of mounting is that which has been generally adopted, and the consequence has been that every now and then complaints have justly been made of the untruthfulness--owing to this particular distortion--of photographs; productions whose chief merit has often been asserted to consist in their absolute truthfulness. This distortion is very manifest when, in a set of portraits, some of the prints happen to have been made in one direction of the paper, and others with the long grain the other way. I have known a case where a proof happened to increase the face in width, and all the other prints increased it in length. Of course, neither was correct, but the proof had been accepted and liked, and the remainder of the set had to be reprinted with the grain of the paper running in the same direction as that in the first one which had been supplied.

Another evil arising from mounting prints while expanded with moisture is, that in drying the contraction of the paper pulls round the card into a curved form and although by rolling this curvature may be temporarily got rid of, the fiber of the paper is in a strained condition, and the bent state of the mount is, sooner or later, renewed thereby.

To remedy these evils it has been proposed to mount the print when dry, by forcible pressure against a slightly damped card, the back of the print having been previously coated with a cement and dried. This plan is, to a great extent, successful; but that it does not give absolute immunity from distortion is, I think, evident from the following consideration. The prints, after being mounted a few days, will show a certain tendency to curl inward. This curling, I take it, is a measure of the strain upon the print, produced by the more complete return to its original dimensions of the paper photograph. Probably it would be well to keep the prints a few days after drying, or to subject them to alternations of damp and dryness, in order to facilitate this complete return before being placed upon the card. The evil of distortion is, however, very slight--perhaps imperceptible--compared with that existing when the prints are mounted wet. I may mention, _en passant_, that I have found gum much more satisfactory as a mountant than starch paste in what is known as the "dry mounting" system.

The paper which has recently been introduced for producing prints by development upon a gelatine surface does not generally, when dried in the usual way, give so good or so brilliant a surface as that of albumenized paper; but on the other hand it is very easy with it to obtain what is called an enamel surface, by simply allowing it to dry in contact with a prepared surface of glass. This method of finishing has therefore been much recommended and adopted, but without consideration of the effect of distortion in connection with it. In an ordinary photograph the print is mounted damp, but in the case of a print squeegeed on to the glass, the paper is saturated and thoroughly swollen, and the use of the squeegee strains it out to its fullest extent. By drying in the position in which it has been held by contact with the glass, the distortion becomes fixed, and if the print is mounted while in this state the distortion is made permanent. How long the strain and distortion remain in an unmounted print, and whether by time and alternations of moisture and dryness the strain would be lost, and if so, whether the brilliant enamel surface would go at the same time, are questions worthy of further investigation and discussion.

For mounting prints upon developed gelatine paper, it has been recommended to cement the edges only, so as to leave the greater part of the print with its enamel surface. This plan is unsatisfactory, for two reasons, besides the objection on the ground of distortion. There is a rough-looking margin which spoils the continuity of appearance, especially (as in the specimens I have seen) where the line of cement is not kept at an exact width, but encroaches here and there.

Secondly, the print, from not being attached to the mount all over, is apt, especially when in a large size, to be somewhat wavy and wanting in flatness. Another plan recommended, as giving a surface resembling albumen paper, is to paste the back of the print without moistening the surface, and so mount. Some prints that have been shown thus treated had so strongly curled the cards upon which they were mounted that it is evident there was considerable strain and consequent distortion.

A third plan recommended is to paste the back of the print while in contact with the glass upon which it has to dry; and, when dried, to mount by passing through a rolling press with a damped card. This plan looks, at first sight, like that recommended for albumen paper, and called "dry" mounting. Consideration, however, will show that there is a radical difference. In the case of the albumen paper the print has been dried without strain, and therefore but little change is to be looked for, while the print dried in contact with glass is strained to the utmost, causing present distortion and future curling of the mount. Perhaps the evil of distortion caused by enameling may be reduced to a minimum by soaking the print in alcohol previous to laying it upon the glass.

Since the distortion of the photograph arises from the unequal expansion of the paper when wet, it becomes a question whether something may not be done in the selection of the paper itself. It may be that some makes vary much less than others in the "length against width" extension of the surface by wetting. It must be remembered that for gelatine emulsion we are not nearly so limited in the selection of paper as when it is required to be albumenized. In the latter case the image is in the paper, whereas with gelatine the image is contained in the surface coating. I may mention that the best plain, i.e., not enameled, but resembling that of ordinary albumen paper, surface that I have seen upon gelatine paper was upon some foreign post that I had obtained for another purpose. The emulsion employed was that described by Mr. J.B.B. Wellington, and this gentleman agreed with me in attributing the superiority of the surface obtained to the fine quality of the paper upon which the emulsion had been coated. Some commercial samples appear to be coated upon paper of somewhat coarse texture. This does not show when the print is enameled.

The unequal expansion of paper is a subject of interest, not only in connection with gelatine paper for development, but with various photographic processes. In making carbon transparencies for instance, the gelatine film which is squeegeed against the glass necessarily takes its dimensions from the paper to which it is attached, and if that be expanded more in the one direction than another, the transparency is similarly deformed; and so, of course, is any negative, enlarged or otherwise, produced in the camera therefrom. A reproduced negative by contact printing may either have the distortion due to expansion of the paper bearing the gelatine film removed or doubled, according to the direction in which the paper is used for the new negative.--_W.E. Debenham, in Br. Jour. of Photography_.

* * * * *

MEASURING THE THICKNESS OF BOILER PLATES.

An ingenious process for determining the thickness of iron plates in boilers, or places where they cannot otherwise be measured without cutting them, has been invented by M. Lebasteur. He spreads upon the plate the thickness of which he desires to find, and also upon a piece of sheet iron of known thickness, a layer of tallow about 0.01 inch thick. He then applies to each, for the same length of time, a small object, such as a surgeon's cauterizing instrument, heated as nearly as possible to a constant temperature. The tallow melts, and as in the thicker plate the heat of the cautery is conducted away more rapidly, while in the thin plate the heat is less freely conducted away, and the tallow is consequently melted over a large area, the diameters of the circles of bare metal around the heated point, bounded after cooling by a little ridge of tallow, will be to each other inversely as the thickness of the plates. The process is stated to have given in the inventor's hands, results of great accuracy.

* * * * *

GROUPS OF STATUARY FOR THE PEDIMENT OF THE HOUSE OF PARLIAMENT IN VIENNA.

The pediment of the central pavilion and the two side pavilions of the new House of Parliament, at Vienna, are to be ornamented with groups of statuary. The group in the middle pediment represents the granting of the constitution by the Emperor Francis Joseph, and was executed by Professor Helmer.

The pediment of the left wing is ornamented by a group representing Justice, and the pediment of the right wing by a group representing the Home Government.

Johannes Benk, the well known Austrian sculptor, designed and executed the last mentioned group. The two figures at the left hand end of this group represent Science and Literature, and those at the right hand end, Industry and Commerce. The entire group consists of nine figures, the middle figure being seated and the rest standing, sitting, and lying, as the space in the pediment allows.

A seated female figure studying a papyrus roll represents Science, and the adjacent female figure, resting one arm on the figure representing Science, and the other, on a lyre, represents Literature or Poetry.

Industry is represented by a strong and powerful woman holding a hammer, and the figure of Mercury and the prow of a vessel represent Commerce.

The modulation and formation of each figure conform strictly to Grecian models, as does also the entire arrangement of the figures in the group; and yet there is much of modern life in the figures, especially in the faces, in which the stereotyped Grecian profile has not been adopted. The attitudes of the figures are also freer and more easy than those of the Grecian period.--_Illustrirte Zeitung_.

* * * * *

ON THE FRITTS SELENIUM CELLS AND BATTERIES.

[Footnote: Paper read before the American Association for the Advancement of Science, at Philadelphia, Sept, 5, 1884.]

By C.E. FRITTS, 42 Nassau St., New York, N.Y.

In all previous cells, so far as I am aware, the two portions or parts of the selenium at which the current enters and leaves it have been in substantially the same electrical state or condition. Furthermore, the paths of the current and of the light have been transverse to each other, so that the two forces partially neutralize each other in their action upon the selenium. Lastly, the current flows through not only the surface layer, which is acted upon by the light, but also the portion which is underneath, and not affected thereby, and which therefore detracts from the actual effect of the light upon the selenium at the surface.

My form of cell is a radical departure from all previous methods of employing selenium, in all of these respects. In the first place, I form the selenium in very thin plates, and polarize them, so that the opposite faces have different electrical states or properties. This I do by melting it upon a plate of metal with which it will form a chemical combination, sufficient, at least, to cause the selenium to adhere and make a good electrical connection with it. The other surface of the selenium is not so united or combined, but is left in a free state, and a conductor is subsequently applied over it by simple contact or pressure.