Chapter XXXVI: The Camera Obscura: How to Make and Use It

BY GORDON STABLES, C.M., M.D., R.N.

Did you never, reader, have a peep in beneath the black cloth where the artist hides his head while he is focusing a sitter for his photograph? I’m sure that many of you have. And what did you see? Why, a pretty little picture in colours of your friend sitting in the chair, laughing like a tramp at a twopenny roll, only upside down. And you have said to yourselves, What a pity it won’t come out in bright colours like that, and why in all the world should it be upside down?

Now I will answer this question before going any further, because it has a bearing on the subject before us--the making of a handy and cheap camera obscura, which cannot fail to be a source of amusement and pleasure, especially when the sun shines.

The reason why the object on the photographer’s ground-glass plate is seen upside down is easily explained. Light, as I need hardly remind older boys, proceeds in straight lines from any illuminated object. It is thus thrown upon the photographer’s plate. A glance at the accompanying diagram (Fig. 1) will suffice to show what I mean.

Let A B be the object--say, an arrow--under consideration, and C D a side view of the ground-glass plate on which the picture is seen. Passing, therefore, in straight lines, the light and colour from the point A will fall at _a_, will they not? and those from B at _b_, and so on from every portion of the arrow, so that the representation therefore on the object-glass will be upside down, or reversed. Q.E.D.

Now about the camera. No one can be said to have invented it, for it is constituted upon the firm and immutable laws of Nature. Roger Bacon is credited with having known this principle. Very likely he did, but he put it to no practical use, though over four hundred years after his time Giovanni Baptiste Porta did. But who knows that the ancients hundreds of years before the Flood were unacquainted with it? Here, for example, is a story a little bird told me one beautiful summer’s day while reclining on the greensward in my woodland study: I had been reading under the shade of my great oak-tree. The sun was very bright, and patches of its light penetrated even through the dark-green branches and fell on my face. Probably it was that which set me a-thinking about the laws of optics and the camera obscura and camera lucida. Suddenly close up above me a bird alighted--it was early in the season--and began pouring out the most charming notes.

‘Many people,’ I said to myself, ‘would take that bird to be the nightingale, but I know it is only a black-cap.’

The words were hardly out of my mouth when a saucy little head with a bright bead of an eye peeped round the corner of a twig at me.

‘_Only_ a black-cap!’ said the owner of the head and the eye. ‘I’d have you know, sir, that we black-caps, as you call us, are of a far older family than the nightingales, and that _they_ first learned their wild notes from us, and not _we_ from them!’

‘You know a deal, I dare say,’ I replied. ‘Can you explain this, then? There is a streak of light creeping in from a point among the boughs up there, and falling on my foolscap, and whenever a pigeon, or hawk, or rook flies past away overhead, his image appears on the paper and crosses it, only in a contrary direction.’

‘Foolscap, indeed!’ replied the bird, ‘it is yourself that should be wearing one. The image on your paper is caused by the reflection of the luminous rays from the flying bird. Now,’ continued the black-cap, ‘I’ll tell you about the camera.

‘You know the ancient Egyptians understood everything!’

‘So they supposed,’ I grunted, ‘but----’

‘Don’t interrupt, please. The dungeons of that mysterious land were once upon a time small and dark and dismal in the extreme, and for a very little fault indeed people were thrown therein, perhaps never to leave them alive.’

‘Well, it came to pass that a certain poor man had offended the king, and all his worldly goods were confiscated and he himself was thrown into a cell in a rock. It was not a large one, and its walls were smoothly cemented with a mixture of lime and sand, and some other ingredients known only to the ancient Egyptians. The cell was situated in the side of a hill, with a door at one side which was opened only once a week, to thrust in a pitcher of water and a bundle of cassava root, on which the poor man lived, and to have the cell cleaned out. The only aperture for light and air was a little round hole at the front of the room, too small for even a bird to get through, though bees and moths often entered and kept the prisoner company. But, lo! every day and all day, especially when the sun shone, the rays of light through that aperture brought with them a picture which they painted on the opposite wall. This picture was upside down, but that was but a small drawback, and everything that happened out of doors or in the city beneath was painted on the wall in a marvellous manner. But when the cell door was opened the picture faded away, so the gaoler never saw it.

‘One day the prisoner addressed the gaoler as follows: “Speak unto the king for me, O my son, and tell him, if all my trespasses are forgiven me, and I am taken up out of this loathsome den, I will build for him in his palace a dark room in which he can sit and see all that is going on in the city beneath spread out before him like a great moving picture.”

‘And the gaoler went and spoke to the king in the prisoner’s behalf, and the poor man was brought before the king and set to work in a room of the palace tower. With the aid of workmen he turned the room into a camera obscura, by means of well-placed steel mirrors casting the picture down upon a white concave table.

‘When the king saw it he was greatly astounded and delighted, and ever after that there was no guest about the palace so greatly honoured as the poor man he had but lately thrown into a dungeon.’

I began to rub my eyes after this, and I am hardly sure yet whether the black-cap had really been speaking, or whether I had dropped asleep and been dreaming.

However, this prisoner did nothing more than you could do. I slept, when a boy, in a little turret chamber, which I easily converted into a camera, a description of which I had read in an old book on ‘The Arts and Sciences.’ I had a white screen placed at the proper focus, and a tiny round hole in the shutter, that was all. It was a very primitive arrangement, but pleased me then.

And I believe that most of my readers who are over twelve can make a handy portable camera from the hints I shall now give.

Before you read any further, then, get an empty matchbox, and put it on the table, bottom upwards. Now draw out the drawer of it about half-way. That matchbox is your rough model for the portable camera. Simple, is it not?

The sketch I here append, however (Fig. 2), is not that of a matchbox, but of your portable camera itself, minus its dark shade. The size of this portable camera will depend upon, and be in the ratio of, your own ambition; the perfection of its make will depend upon your own ingenuity.

1. Well, then, you are to make or get made a small box of either very thin wood or very strong pasteboard, covered with thin cloth and painted some dark colour. Size, say, six inches high, six inches wide, and one foot long. This box is open only in front, and therein fits or slips the focusing drawer with its lens.

2. This drawer is also of the same material, and is open at the end that fits into the box, that is the end opposite the lens, and should work easily in or out, and admit no light except through the lens, the magnifying power of which need not be very great.

3. The whole interior of box and drawer is to be stained of a dull black colour.

4. Into the top of the box is let a piece of ground glass, occupying the whole breadth nearly of the top, and two-thirds of its length.

5. Into the box is fitted or let a mirror, which faces the drawer and lies at an angle of forty-five degrees.

Now turn your eyes to Fig. 2, and I will try to explain it. The dimensions of the box are marked in plain figures, as the drapers say. The length of the drawer is about five inches or less. This drawer is represented in the figure as pulled about half-way out. When shut up it will reach the letters C A. The lens is about an inch in diameter, and may be bought cheap at any optician’s, or even fitted for you there.

The piece of ground glass on the top occupies the position marked out by the letters F, E, B, G.

The mirror inside occupies the position indicated by the dotted line A B.

But your camera is not complete yet. You have your dark shade to slip on and fasten. This shade (_vide_ Fig. 3) is a lid, open at both ends, that goes right over two-thirds of the whole box when closed, covering that portion of it seen in Fig. 2 between the squares F, E, G, B and E, H, B, I. This lid is fixed by means of a close-gummed cloth hinge to the box at the dotted line F, E. It is free every way else, so that, when lifted up to an angle of forty-five degrees, it keeps the light away from the ground-glass top, and permits you to see the picture thereon. This shade is, of course, also stained of a dark colour internally.

Now your portable camera is complete. To use it, all you have to do is to hold it in your hand, with the lens turned towards the picture you want represented, and the shade raised, then to pull the focusing-drawer in and out till you have a clear, well-defined picture on your ground-glass top. When the sun is shining brightly the effect is charming; but you yourself and camera ought to be well in the shade.

Now, the instrument I describe is very simple, but its principles may be extended. You might have it on a stand with racks and pulleys for adjustment; and you may have a dark cloth over the shade: the picture will then be ever so much more bright. The great charm of a camera like this is to have a real and lifelike picture in natural colours spread out before you, to see still life as it stands, the trees waving in the wind, and flowers nodding in the sunshine, and every human being or animal that passes walking and moving on your plate.