Chapter XVII: Appendix

The following is extracted from Maxwell’s paper.

The following table contains the means of four sets of observations by the same observer (K.):--

TABLE IV. (K.)

44·3 (20) + 31·0 (44) + 27·7 (68) = W.
16·1 (28) + 25·6 (44) + 30·6 (68) = W.
22·0 (32) + 12·1 (44) + 30·6 (68) = W.
6·4 (24) + 25·2 (36) + 31·3 (68) = W.
15·3 (24) + 26·0 (40) + 30·7 (68) = W.
19·8 (24) + 35·0 (46) + 30·2 (68) = W.
21·2 (24) + 41·4 (48) + 27·0 (68) = W.
22·0 (24) + 62·0 (52) + 13·0 (68) = W.
21·7 (24) + 10·4 (44) + 61·7 (56) = W.
20·5 (24) + 23·7 (44) + 40·5 (60) = W.
19·7 (24) + 30·3 (44) + 33·7 (64) = W.
18·0 (24) + 31·2 (44) + 32·3 (72) = W.
17·5 (24) + 30·7 (44) + 44·0 (76) = W.
18·3 (24) + 33·2 (44) + 63·7 (80) = W.

X.--REDUCTION OF THE OBSERVATIONS.

By eliminating W from the equations above by means of the standard equation, we obtain equations involving each of the fourteen selected colours of the spectrum, along with the three standard colours; and by transposing the selected colour to one side of the equation, we obtain its value in terms of the three standards. If any of the terms of these equations are negative, the equation has no physical interpretation as it stands; but by transposing the negative term to the other side it becomes positive, and then the equation may be verified.

The following table contains the values of the fourteen selected tints in terms of the standards. To avoid repetition, the symbols of the standard colours are placed at the head of each column:--

TABLE VI.

Observer (K.) (24) (44) (68)
44·3 (20) = 18·6 + 0·4 + 2·8
16·1 (28) = 18·6 + 5·8 - 0·1
22·0 (32) = 18·6 + 19·3 - 0·1
25·2 (36) = 12·2 + 31·4 - 0·8
26·0 (40) = 3·3 + 31·4 - 0·2
35·0 (46) = - 1·2 + 31·4 + 0·3
41·4 (48) = - 2·6 + 31·4 + 3·5
62·0 (52) = - 3·4 + 31·4 + 17·5
61·7 (56) = - 3·1 + 21·0 + 30·5
40·5 (60) = - 1·9 + 7·7 + 30·5
33·7 (64) = - 1·1 + 1·1 + 30·5
32·3 (72) = + 0·6 + 0·2 + 30·5
44·0 (76) = + 1·1 + 0·7 + 30·5
63·7 (80) = + 0·3 - 1·8 + 30·5

Mr. James Simpson, formerly student of Natural Philosophy in my class, has furnished me with thirty-three observations taken in good sunlight. Ten of these were between the two standard colours, and give the following result:--

33·7 (88) + 33·1 (68) = W.

The mean errors of these observations were as follows:--

Error of (88) = 2·5; of (68) = 2·3; of (88) + (68)
= 4·8; of (88) - (68) = 1·3.

The fact that the mean error of the sum was so much greater than the mean error of the difference, indicates that in this case, as in all others that I have examined, observations of equality of tint can be depended on much more than observations of equality of illumination or brightness.

From six observations of my own, made at the same time, I have deduced the “trichromic” equation--

22·6 (104) + 26 (88) + 37·4 (68) = W (2)

If we suppose that the light which reached the organ of vision was the same in both cases, we may combine these equations by subtraction, and so find

22·6 (104) - 7·7 (88) + 4·3 (68) = D (3)

where D is that colour, the absence of the sensation of which constitutes the defect of the dichromic eye.

The sensation which I have in addition to those of the dichromic eye is therefore similar to the full red (104), but different from it in that the red (104) has 7·7 of green (88) in it which must be removed, and 4·3 of blue (68) substituted. This agrees pretty well with the colour which Mr. Pole[A] describes as neutral to him, though crimson to others. It must be remembered, however, that different persons of ordinary vision require different proportions of the standard colours, probably owing to differences in the absorptive powers of the media of the eye, and that the above equation (2), if observed by K., would have been

23 (104) + 32 (88) + 31 (68) = W (4)

and the value of D, as deduced from these observers, would have been

23 (104) - 1·7 (88) - 1·1 (68) = D (5)

in which the defective sensation is much nearer to the red of the spectrum. It is probably a colour to which the extreme red of the spectrum tends, and which differs from the extreme red only in not containing that small proportion of “yellow” light which renders it visible to the colour blind.

[A] Philosophical Transactions, 1859, Part I., p. 329.

From other observations by Mr. Simpson the following results have been deduced:--

TABLE A.

(88) (68) | (88) (68)
(99·2 +) = 33·7 1·9 | 100 (96) = 108 7
31·3 (96) = 33·7 2·1 | 100 (92) = 120 5
28 (92) = 33·7 1·4 | 100 (88) = 100 0
33·7 (88) = 33·7 0 | 100 (84) = 61 11
54·7 (84) = 33·7 6·1 | 100 (82) = 47 21
71 (82) = 33·7 15·1 | 100 (80) = 34 33
99 (80) = 33·7 33·1 | 100 (78) = 22 47
70 (78) = 15·7 33·1 | 100 (76) = 10 59
56 (76) = 5·7 33·1 | 100 (72) = 1 92
36 (72) = 0·3 33·1 | 100 (68) = 0 100
33·1 (68) = 0 33·1 | 100 (64) = 0 83
40 (64) = 0·2 33·1 | 100 (60) = 3 60
55·5 (60) = 1·7 33·1 |
(57 -) = 0·3 33·1 |

In the table on the left side (99·2 +) means the whole of the spectrum beyond (99·2) on the scale, and (57 -) means the whole beyond (57) on the scale. The position of the fixed lines with reference to the scale was as follows:--

A, 116; a, 112; B, 110; C, 106; D, 98·3; E, 88; F, 79; G, 61; H, 44.

The values of the standard colours in different parts of the spectrum are given on the right side of the above table, and are represented by the curves of Fig. 9, Plate II., where the left-hand curve represents the intensity of the “yellow” element, and the right-hand curve that of the “blue” element of colour as it appears to the colour blind.

The appearance of the spectrum to the colour blind is as follows:--

From A to E the colour is pure “yellow,” very faint up to D, and reaching a maximum between D and E. From E to one-third beyond F towards G the colour is mixed, varying from “yellow” to “blue,” and becoming neutral or “white” at a point near F. In this part of the spectrum the total intensity, as given by the dotted line, is decidedly less than on either side of it, and near the line F, the retina close to the “yellow spot” is less sensible to light than the parts further from the axis of the eye. This peculiarity of the light near F is even more marked in the colour blind than in the ordinary eye. Beyond F the “blue” element comes to a maximum between F and G, and then diminishes towards H, the spectrum from this maximum to the end being pure “blue.”

The results given above were all obtained with the light of white paper, placed in clear sunshine. I have obtained similar results when the sun was hidden, by using the light of uniformly illuminated clouds, but I do not consider these observations sufficiently free from disturbing circumstances to be employed in calculation. It is easy, however, by means of such observations, to verify the most remarkable phenomena of colour blindness, as, for instance, that the colours from red to green appear to differ only in brightness, and that the brightness may be made identical by changing the width of the slit; that the colour near F is a neutral tint, and that the eye in viewing it sees a dark spot in the direction of the axis of vision; that the colours beyond are all blue of different intensities, and that any “blue” may be combined with any “yellow” in such proportions as to form “white.” These results I have verified by the observations of another colour-blind gentleman, who did not obtain sunlight for his observations; and as I have now the means of carrying the requisite apparatus easily, I hope to meet with other colour-blind observers, and to obtain their observations under more favourable circumstances.

MEASUREMENTS OF COLOUR FIELDS.

Some experiments in the measurement of the colour fields in the horizontal direction with the pure spectrum colours will help to show what importance is to be attached to the luminosity of the colour and the size of the spot of light with which the observations are made. A yellow and a blue of the spectrum were taken of such hues that when mixed they formed a patch of white light similar to the electric light. Their luminosities were measured, and the yellow found to be 1·6 of the light of an amyl-acetate lamp or 1·28 standard candles; the blue was 1/24 of this luminosity. The fields for these two colours were measured by automatically throwing spots of each colour separately on a white card which moved round a centre over which the eye was placed. The light was subsequently diminished to ½, ¼, and ⅛ of the above values, and readings again made. The following results were obtained with a spot of ·7 inch diameter:--

Yellow. Blue.
Light. -------------------------- ---------------------------
Nasal side. Temporal side. Nasal side. Temporal side.

Full 33° 45° 35° 45°
½ 24° 36° 26° 38°
¼ 18° 24° 22° 32°
⅛ 11° 15° 19° 30°

With a spot of ·3 inch diameter the following were obtained:--

Full 24° 32° 21° 27°
½ 17° 28° 16° 22°
¼ 13° 16° 14° 20°
⅛ 8° 10° 13° 16°

It will be evident how the field contracts as the light is diminished in brightness, and also that the blue field does not diminish equally with the yellow field, but is more persistent. Again, it will be noticed that the luminosity of the blue, for the same extent of field to be covered, has to be much lower than for the yellow.

The diminished area of the spot of light also diminishes the field, and the same order of diminution of field is obtained as with the larger spot.

Another set of experiments, made with the same aperture of slit passed through the spectrum, and the field taken at different points, give the following results:--

Spectrum scale. Nasal side. Temporal side.
(See Fig. 41,
page 210.)

58·6 18° 35°
54·6 27° 46°
50·6 33° 47°
46·6 25° 30°
42·6 21° 21°
38·6 17° 17°
34·6 22° 30°
30·6 25° 33°
26·6 33° 40°
22·6 37° 44°
18·6 28° 40°
14·6 22° 34°
8·6 20° 30°

Here we see that although the luminosity of the colour spots varies at the spectrum luminosity, the fields do not vary proportionally; when the luminosities of the green, yellow and red are made equal, the fields become nearly equal on the nasal side. The field for the blue, however, then becomes vastly larger than that for the others, showing a peculiarity which is very remarkable.

_Spectrum Scale._]

Recently published experiments on colour fields have been so largely based on the exigencies of the Hering theory, that it is somewhat difficult to decide their significance from any other aspect.

TABLE I.--LUMINOSITY CURVES FOR THE NORMAL EYE (see Fig. 20).

-------+--------------+----------------+--------------+-----------
I. | II. | III. | IV. | V.
-------+--------------+----------------+--------------+-----------
Scale | Wave-length. | Outside yellow | Yellow spot. | Fovea
number.| | spot. | | centralis.
-------+--------------+----------------+--------------+-----------
64 | 7217 | | |
63 | 7082 | .. | 1 |
62 | 6957 | 1 | 2 | 2
61 | 6839 | 2 | 4 | 4
60 | 6728 | 3·5 | 7 | 8
59 | 6621 | 7·5 | 12·5 | 15·5
58 | 6520 | 12·5 | 21 | 24
57 | 6423 | 19 | 33 | 37·5
56 | 6330 | 27·5 | 50 | 60
55 | 6242 | 35 | 65 | 77
54 | 6152 | 43 | 80 | 90
53 | 6074 | 52·5 | 90 | 97
52 | 5996 | 61·0 | 96 | 100
51 | 5919 | 71·0 | 99 | 100
50 | 5850 | 79·0 | 100 | 98
49 | 5783 | 84 | 99 | 95
48 | 5720 | 85 | 97 | 90
47 | 5658 | 83·5 | 92·5 | 85
46 | 5596 | 81·0 | 87 | 79
45 | 5538 | 77·0 | 81 | 72·5
44 | 5481 | 72.5 | 75 | 66
43 | 5427 | 68·0 | 69 | 59
42 | 5373 | 62·5 | 62·5 | 51
41 | 5321 | 57 | 57 | 45
40 | 5270 | 52 | 50 | 40
39 | 5221 | 46 | 42·5 | 32
38 | 5172 | 41·5 | 36 | 27·5
37 | 5128 | 37·5 | 29·5 | 22·0
36 | 5085 | 33·5 | 24 | 18
35 | 5043 | 30·0 | 18·2 | 14
34 | 5002 | 26·5 | 14·2 | 10
33 | 4963 | 24 | 10·5 | 8·4
32 | 4924 | 21 | 8·5 | 6·5
31 | 4885 | 18·5 | 7·0 | 5·5
30 | 4848 | 16·5 | 5·5 | 4·0
29 | 4812 | 14·5 | 4·7 | 3·5
28 | 4776 | 13·0 | 4·0 | 3·0
27 | 4742 | 11·5 | 3·5 | 2·0
26 | 4707 | 10·5 | 2·8 | 2·4
25 | 4675 | 9·4 | 2·3 | 2·1
24 | 4639 | 8·2 | 1·82 | 1·9
23 | 4608 | 7·3 | 1·6 | 1·5
22 | 4578 | 6·3 | 1·4 |
21 | 4548 | 5·7 | 1·2 |
20 | 4517 | 5·0 | 1·08 | 1·0
19 | 4488 | 4·5 | ·94 |
18 | 4459 | 4·0 | ·86 |
17 | 4437 | 3·6 | ·78 |
16 | 4404 | 3·1 | ·70 |
15 | 4377 | 2·7 | ·62 | ·62
14 | 4349 | 2·3 | ·56 |
13 | 4323 | 2·1 | ·50 |
12 | 4296 | 1·9 | ·45 |
11 | 4271 | 1·65 | ·40 |
10 | 4245 | 1·4 | ·34 |
9 | 4221 | 1·2 | ·30 |
8 | 4197 | 1·0 | ·26 |
7 | 4174 | ·88 | ·22 |
6 | 4151 | ·75 | ·18 |
5 | 4131 | ·63 | ·16 |
4 | 4106 | ·50 | ·14 |
-------+--------------+----------------+--------------+-----------

TABLES II. AND III.--CURVES OF LUMINOSITY OF A PARTIALLY RED-BLIND AND OF A PARTIALLY GREEN-BLIND PERSON (see Fig. 23).

-------------+------------+-----------------------
| | Luminosity.
Scale number.|Wave-length.+----------+------------
| |Red-blind.|Green-blind.
-------------+------------+----------+------------
64 | 7217 | 0 | 1
62 | 6957 | 1 | 2
60 | 6728 | 2 | 7
58 | 6520 | 6 | 21
56 | 6330 | 12 | 50
54 | 6152 | 26 | 80
52 | 5996 | 49 | 96
50 | 5850 | 70 | 98
48 | 5720 | 77 | 93
46 | 5596 | 77 | 83
44 | 5481 | 70 | 70
42 | 5373 | 61 | 55
40 | 5270 | 47 | 40
38 | 5172 | 34 | 27
36 | 5085 | 23 | 18
34 | 5002 | 14 | 10
32 | 4924 | 8·5 | 5·5
30 | 4848 | 5·5 | 3·0
28 | 4776 | 4·0 | 2·5
26 | 4707 | 2·7 | 2·0
24 | 4639 | 1·8 | 1·8
22 | 4578 | 1·35 | 1·4
20 | 4517 | 1·1 | 1·1
-------------+------------+----------+------------

TABLE IV.--LUMINOSITY OF SPECTRUM REDUCED IN INTENSITY, SO THAT D = 1/132·5 AMYL LAMP 1 FOOT DISTANT (see Fig. 25).

-------+---------+-----------+------------+---------------
| | Mean | | Persistency
| | reading, | P. and Q.’s| curve for the
Scale | Mean | reduced to| readings, | centre of
number.| reading.| 100 max. | 100 max. | the eye.
-------+---------+-----------+------------+---------------
55·6 | ·5 | ·6 | 2 | 2
53·6 | 5·5 | 7·0 | 3·6 | 3·6
51·6 | 13 | 16·7 | 8 | 8
49·6 | 23 | 29·7 | 22 | 22
47·6 | 40 | 50·0 | 44 | 44
45·6 | 57 | 71·2 | 69 | 69
43·6 | 70 | 87·5 | 93 | 93
41·6 | 79 | 98·7 | 100 | 99·5
39·6 | 78 | 97·5 | 99·5 | 98·5
37·6 | 74 | 92·5 | 96 | 93
35·6 | 66 | 82·5 | 89 | 84
33·6 | 55 | 68·7 | 77·5 | 71
31·6 | 44·5 | 55·2 | 61 | 53·5
29·6 | 35 | 43·7 | 45·5 | 36·5
27·6 | 24 | 30·0 | 33·5 | 24
25·6 | 17 | 21·7 | 25 | 16
23·6 | 13 | 16·7 | 18 | 10
21·6 | 10 | 12·5 | 13 | 8
19·6 | 8 | 10·0 | 9·5 | 6
13·6 | 3 | 3·7 | 4·2 | 3
9·6 | 2 | 2·5 | 2·5 | 2
-------+---------+-----------+------------+---------------

TABLE V.--LIMIT OF COLOUR VISION (see Fig. 26.).

--------+---------+---------------+------------+----------------
Scale | Wave- | Mean reading | Luminosity | Luminosity of
Number. | Length. | of the colour | of the | the rays when
| | limit of the | ordinary | each colour
| | spectrum D, | spectrum. | disappears,
| | being 1 amyl | | each ray having
| | lamp in | | the original
| | 1/10000ths. | | luminosity of
| | | | 1 amyl lamp
| | | | in 1/100000ths.
--------+---------+---------------+------------+----------------
61 | 6839 | 120 | 4 | 48·0
60 | 6728 | 67 | 7 | 46·9
58 | 6520 | 26 | 21 | 54·6
56 | 6330 | 13 | 50 | 65·0
54 | 6152 | 9·5 | 80 | 76·0
52 | 5996 | 9·0 | 96 | 86·4
50 | 5850 | 9·0 | 100 | 90·0
48 | 5720 | 9·0 | 97 | 87·3
44 | 5481 | 9·5 | 75 | 71·3
40 | 5270 | 10·5 | 50 | 52·5
36 | 5085 | 12·5 | 24 | 30·0
32 | 4924 | 18 | 8·5 | 15·3
28 | 4776 | 32 | 4·0 | 12·8
24 | 4639 | 55 | 1·8 | 12·0
20 | 4517 | 90 | 1·08 | 9·7
16 | 4404 | 160 | ·70 | 11·2
12 | 4296 | 250 | ·45 | 11·0
8 | 4197 | 400 | ·26 | 10·4
4 | 4106 | 700 | ·14 | 9·8
--------+---------+---------------+------------+----------------

TABLE VI.--EXTINCTION BY CENTRAL PORTION OF NORMAL EYE (see Fig. 28).

-------+------------+--------------+-----------+----------+-----------
I. | II. | III. | IV. | V. | VI.
-------+------------+--------------+-----------+----------+-----------
| | E. | L. | |Persistency
| | Reduction of | | | curve
| | original | Luminosity|(E × L) / | 650 / E
Scale |Wave-length.| luminosity | of | 100 |(Maximum =
number.| | in millionths| original | | 100).
| | to cause | beam. | |
| | extinction. | | |
-------+------------+--------------+-----------+----------+-----------
64 | 7217 | 55,000 | | |
63 | 7082 | 30,000 | 1 | 300·0 |
62 | 7957 | 15,000 | 2 | 300·0 |
61 | 6839 | 7500 | 4 | 300·0 |
60 | 6728 | 3750 | 7 | 262·5 |
59 | 6621 | 1900 | 12·5 | 237·5 | ·34
58 | 6520 | 1050 | 21 | 220·5 | ·62
57 | 6423 | 650 | 33 | 214·5 | 1·0
56 | 6333 | 380 | 50 | 190·0 | 1·71
55 | 6242 | 272 | 65 | 176·8 | 2·38
54 | 6152 | 196 | 80 | 156·0 | 3·32
53 | 6074 | 140 | 90 | 126·0 | 4·64
52 | 5996 | 97 | 96 | 93·12 | 6·70
51 | 5919 | 57 | 99 | 56·43 | 11·40
50 | 5850 | 35 | 100 | 35·0 | 18·6
49 | 5783 | 24 | 99 | 23·76 | 27·1
48 | 5720 | 17 | 97 | 16·49 | 38·2
47 | 5658 | 12·6 | 92·5 | 11·65 | 51·6
46 | 5596 | 10·2 | 87 | 8·87 | 63·7
45 | 5538 | 8·6 | 81 | 6·97 | 75·6
44 | 5481 | 7·4 | 75 | 5·55 | 87·8
43 | 5427 | 6·7 | 69 | 4·62 | 97·0
42 | 5373 | 6·55 | 62·5 | 4·09 | 99·5
41 | 5321 | 6·5 | 57 | 3·705 | 100
40 | 5270 | 6·55 | 50 | 3·27 | 98·5
39 | 5221 | 6·65 | 42·5 | 2·83 | 97·5
38 | 5172 | 6·85 | 36 | 2·46 | 95·0
37 | 5128 | 7·2 | 29·5 | 2·12 | 90·0
36 | 5085 | 7·6 | 24 | 1·82 | 81·3
35 | 5043 | 8·15 | 18·2 | 1·48 | 80·0
34 | 5002 | 8·8 | 14·2 | 1·25 | 74·0
33 | 4963 | 10·2 | 10·5 | 1·07 | 63·0
32 | 4924 | 11·6 | 8·5 | ·988 | 56·0
31 | 4885 | 13·6 | 7·0 | ·952 | 47·7
30 | 4848 | 16·3 | 5·5 | ·896 | 40·0
29 | 4812 | 20·5 | 4·7 | ·963 | 31·7
28 | 4776 | 26·0 | 4·0 | 1·040 | 25·0
27 | 4742 | 31·0 | 3·5 | 1·085 | 20·9
26 | 4707 | 38·5 | 2·8 | 1·078 | 16·9
25 | 4674 | 46·0 | 2·3 | 1·058 | 14·1
24 | 4639 | 56·0 | 1·82 | 1·019 | 11·6
23 | 4608 | 67·0 | 1·6 | 1·072 | 9·7
22 | 4578 | 80 | 1·4 | 1·120 | 8·41
21 | 4548 | 95 | 1·2 | 1·140 | 7·22
20 | 4517 | 107 | 1·08 | 1·156 | 6·1
19 | 4488 | 124 | ·94 | 1·165 | 5·23
18 | 4459 | 140 | ·86 | 1·204 | 4·64
17 | 4437 | 160 | ·78 | 1·228 | 4·1
16 | 4404 | 180 | ·70 | 1·260 | 3·60
15 | 4377 | 200 | ·62 | 1·240 | 3·25
14 | 4349 | 220 | ·56 | 1·232 | 2·95
13 | 4323 | 240 | ·50 | 1·200 | 2·7
12 | 4296 | 270 | ·45 | 1·215 | 2·4
11 | 4271 | 300 | ·40 | 1·200 | 2·18
10 | 4245 | 335 | ·34 | 1·139 | 1·94
9 | 4221 | 375 | ·30 | 1·125 | 1·73
8 | 4197 | 430 | ·26 | 1·118 | 1·51
7 | 4174 | 490 | ·22 | 1·078 | 1·32
6 | 4151 | 510 | ·18 | ·918 | 1·27
5 | 4131 | 640 | ·16 | 1·024 | 1·01
4 | 4106 | 750 | ·14 | 1·050 | 0·86
-------+------------+--------------+-----------+----------+-----------

TABLE VII.--EXTINCTION BY WHOLE EYE

(see Fig. 28).

--------+---------+--------------+------------+---------+------------
I. | II. | III. | IV. | V. | VI.
--------+---------+--------------+------------+---------+------------
Scale | Wave- | E. | L. |(E × L) /| Persistency
number. | length. | Reduction of | | 160 | curve
| | original | Luminosity | | 650 / E
| | luminosity | of | | (Maximum=
| | in millionths| original | | 100).
| | to cause | beam. | |
| | extinction. | | |
--------+---------+--------------+------------+---------+------------
38 | 5172 | 6·9 | 41·5 | 2·86 | 94·2
37 | 5128 | 7·1 | 37·5 | 2·66 | 91·6
36 | 5085 | 7·4 | 33·5 | 2·48 | 87·8
35 | 5043 | 7·7 | 30·0 | 2·31 | 84·4
34 | 5002 | 8·0 | 26·5 | 2·12 | 81·2
33 | 4963 | 8·4 | 24·0 | 2·02 | 77·5
32 | 4924 | 8·8 | 21·0 | 1·85 | 73·8
31 | 4885 | 9·4 | 18·5 | 1·74 | 69·2
30 | 4848 | 10·0 | 16·5 | 1·65 | 65·0
29 | 4812 | 10·7 | 14·5 | 1·55 | 60·6
28 | 4776 | 11·5 | 13·0 | 1·49 | 56·5
27 | 4742 | 13·0 | 11·5 | 1·49 | 50·0
26 | 4707 | 14·5 | 10·5 | 1·52 | 44·8
24 | 4639 | 18·5 | 8·2 | 1·52 | 34·1
22 | 4578 | 23·0 | 6·3 | 1·45 | 28·3
20 | 4517 | 30·0 | 5·0 | 1·50 | 21·7
18 | 4459 | 39·0 | 4·0 | 1·56 | 16·7
16 | 4404 | 51 | 3·1 | 1·59 | 12·3
14 | 4349 | 66 | 2·3 | 1·52 | 9·85
12 | 4296 | 80 | 1·9 | 1·52 | 8·12
10 | 4245 | 110 | 1·4 | 1·54 | 5·91
8 | 4197 | 154 | 1·0 | 1·54 | 4·22
6 | 4151 | 204 | ·75 | 1·54 | 3·18
4 | 4106 | 307 | ·5 | 1·54 | 2·11
2 | 4063 | 513 | ·3 | 1·54 | 1·26
0 | 4020 | 770 | ·2 | 1·54 | ·84
--------+---------+--------------+------------+---------+------------

From 38 to 64 the extinction is the same as with the central part of the eye.

TABLE VIII.--P.’S CURVES[B] (see Fig. 31).

-------+--------+------------+------------+-------------+-----------+------------
I. | II. | III. | IV. | V. | VI. | VII.
-------+--------+------------+------------+-------------+-----------+------------
Scale | Wave- | Mean | Adopted | Persistency | P.’s | Absolute
number.| length.| reading of | reading in | curve | luminosity| luminosity
| | extinction | millionths | (680 / | curve. | of
| | in | of |ad. reading).| | extinction.
| | millionths | original | | | (IV.×VI.) /
| | of | luminosity.| | | 14
| | original | | | |
| | luminosity.| | | |
-------+--------+------------+------------+-------------+-----------+------------
52 | 5996 | 68 | 68 | 10 | 7 | 34
50 | 5850 | 35 | 35 | 19·4 | 19 | 47·5
48 | 5720 | 17 | 17 | 40 | 39 | 47·3
46 | 5596 | 10·2 | 10 | 68 | 65 | 46·4
45 | 5538 | 9·3 | 9·0 | 76 | 76 | 48·8
44 | 5481 | 8·0 | 8·1 | 84 | 90 | 52·8
42 | 5373 | 7·2 | 7·2 | 94·5 | 98 | 50·3
40 | 5270 | 6·7 | 6·8 | 100 | 99 | 48·1
38 | 5172 | 7·2 | 7·0 | 97 | 97·5 | 48·7
36 | 5085 | 8·05 | 7·7 | 90 | 90 | 49·5
34 | 5002 | 8·05 | 8·4 | 81 | 80 | 47·9
32 | 4924 | 9·9 | 9·8 | 69 | 65 | 45·5
30 | 4848 | 13·2 | 12·5 | 54 | 50 | 44·6
28 | 4776 | 13·9 | 15·0 | 45·3 | 36 | 38·6
27 | 4742 | 16·8 | 17·0 | 40 | 31·5 | 38·2
26 | 4707 | 21·6 | 20·5 | 32 | 26·5 | 38·8
24 | 4639 | 30 | 27 | 25 | 19·5 | 37·6
22 | 4578 | 36 | 35 | 19 | 14 | 35
20 | 4517 | 42 | 45 | 15·5 | 10 | 32·2
16 | 4404 | 79 | 79 | 8·5 | 5·5 | 31·2
10 | 4245 | 180 | 190 | 3·6 | 2·5 | 32·2
6 | 4151 | 270 | 270 | 2·7 | |
-------+--------+------------+------------+-------------+-----------+------------

[B] In this and the next two Tables the intensity of the
illumination of the D ray before reduction is equal to that
of an amyl-acetate lamp at one foot from a screen. The
figures in Col. VII. are in millionths of the illumination
of an amyl-acetate lamp at one foot distant, every ray
being made of that intensity.

TABLE IX.--H. R.’S CURVES (see Fig. 32).

-------+--------+------------+------------+-------------+-----------+------------
I. | II. | III. | IV. | V. | VI. | VII.
-------+--------+------------+------------+-------------+-----------+------------
Scale | Wave- | Mean | Adopted | Persistency | Luminosity| Absolute
number.| length.| reading of | reading in | curve | curve. | luminosity
| | extinction | millionths | (590 / | | of
| | in | of |ad. reading).| | extinction
| | millionths | original | | | (IV.×VI.) /
| | of | luminosity.| | | 48
| | original | | | |
| | luminosity.| | | |
-------+--------+------------+------------+-------------+-----------+------------
57 | 6423 | 1200 | 1200 | ·49 | 5 | 125
56 | 6330 | 900 | 850 | ·69 | 7 | 124
55 | 6242 | 500 | 550 | 1·07 | 10 | 115
54 | 6152 | 250 | 250 | 2·36 | 17 | 88
53 | 6074 | .. | 150 | 3·93 | 25 | 78
52 | 5996 | 90 | 90 | 6·56 | 35 | 66
51 | 5919 | 60 | 45 | 13·1 | 47 | 44
50 | 5850 | 27 | 27 | 21·8 | 57 | 32
48 | 5720 | 18 | 15 | 39·3 | 66 | 21
46 | 5596 | 10 | 10 | 59 | 69 | 14
44 | 5481 | 9·3 | 8 | 73·8 | 64 | 11
42 | 5373 | 6·5 | 6·2 | 95·1 | 56·5 | 7
40 | 5270 | 5·9 | 5·9 | 100 | 45 | 5·5
38 | 5172 | 6 | 6 | 98·3 | 32 | 4
36 | 5085 | .. | 6·6 | 89·4 | 20 | 2·7
35 | 5043 | 7 | 7·2 | 81·9 | 16 | 2·4
34 | 5002 | .. | 8 | 73·8 | 12·5 | 2·1
32 | 4924 | 10 | 9·6 | 61·5 | 8 | 1·6
30 | 4848 | 11·5 | 12 | 49·2 | 6 | 1·5
28 | 4776 | 14·5 | 14·5 | 40·7 | 5 | 1·5
26 | 4707 | 20 | 17·5 | 33·7 | 4 | 1·5
24 | 4639 | 20 | 22 | 26·8 | 3 | 1·4
22 | 4578 | .. | 30 | 19·7 | 2·4 | 1·5
18 | 4459 | 55 | 57 | 10·4 | 1·3 | 1·5
14 | 4349 | 115 | 115 | 5·1 | ·7 | 1·7
10 | 4245 | .. | 160 | 3·7 | ·5 | 1·7
6 | 4151 | 200 | 200 | 2·9 | ·4 | 1·7
-------+--------+------------+------------+-------------+-----------+------------

TABLE X.--V. H.’S CURVES (see Fig. 33).

-------+--------+------------+------------+-------------+-----------+------------
I. | II. | III. | IV. | V. | VI. | VII.
-------+--------+------------+------------+-------------+-----------+------------
Scale | Wave- | Mean | Adopted | Persistency | Luminosity| Absolute
number.| length.| reading of | reading in | curve | curve. | luminosity
| | extinction | millionths | (530 / | | of
| | in | of |ad. reading).| | extinction
| | millionths | original | | | (IV.×VI.) /
| | of | luminosity.| | | 75.
| | original | | | |
| | luminosity.| | | |
-------+--------+------------+------------+-------------+-----------+------------
57 | 6423 | 500 | 500 | 1·1 | 31 | 206
56 | 6330 | 350 | 350 | 1·5 | 43 | 200
54 | 6152 | 200 | 180 | 2·9 | 61 | 146·4
52 | 5996 | 100 | 100 | 5·3 | 70 | 93·3
50 | 5850 | 40 | 40 | 13·3 | 73 | 38·9
48 | 5720 | .. | 25 | 21·2 | 69 | 23
46 | 5596 | 10 | 10 | 53·0 | 63 | 8·4
45 | 5538 | 6·5 | 6·5 | 81·6 | 58 | 5·0
44 | 5481 | 6·0 | 5·7 | 93 | 54 | 4·1
42 | 5373 | 5·5 | 5·3 | 100 | 46 | 3·3
40 | 5270 | 5·5 | 5·4 | 98·2 | 36 | 2·6
38 | 5172 | 5·7 | 5·7 | 93 | 24 | 1·8
36 | 5085 | 6·7 | 6·5 | 81·6 | 15 | 1·3
34 | 5002 | 7·0 | 7·0 | 75·7 | 9·5 | ·89
32 | 4924 | 8·5 | 8·5 | 62·3 | 7·0 | ·79
30 | 4848 | 10·7 | 10·5 | 50·5 | 5·0 | ·70
28 | 4776 | 16 | 16 | 33·1 | 3·7 | ·79
26 | 4707 | .. | 22·5 | 23·5 | 2·7 | ·81
24 | 4639 | 30 | 31 | 17·1 | 1·82 | ·75
22 | 4578 | 42·5 | 42 | 12·6 | 1·4 | ·78
20 | 4517 | 55 | 55 | 9·6 | 1·0 | ·73
16 | 4404 | 105 | 100 | 5·3 | ·7 | ·93
12 | 4296 | 175 | 170 | 3·1 | ·45 | 1·02
10 | 4245 | 200 | 200 | 2·7 | ·34 | ·91
-------+--------+------------+------------+-------------+-----------+------------

TABLE XI.--B. C.’S CURVES (see Fig. 34).

--------+--------+-------------+------------+-----------+-------------
| | | | |
I. | II. | III. | IV. | V. | VI.
--------+--------+-------------+------------+-----------+-------------
Scale | Wave- | Adopted | Persistency| Luminosity| Absolute
Number. | length.| reading in | curve | of | luminosity
| | hundred | 12,500 | original | of
| | thousandths.| readings | beam. | extinction
| | | in V. | | III. and V.
--------+--------+-------------+------------+-----------+-------------
61 | 6839 | 7500 | 1·6 | |
60 | 6728 | 5500 | 2·3 | ·5 | 27·5
59 | 6622 | 4000 | 3·1 | 1 | 40
58 | 6520 | 2800 | 4·5 | 2 | 56
57 | 6423 | 2000 | 6·2 | 4 | 80
56 | 6330 | 1500 | 8·3 | 6 | 90
55 | 6242 | 1150 | 10·8 | 8 | 92
54 | 6152 | 950 | 13·1 | 11·5 | 109·2
53 | 6074 | 750 | 16·6 | 16 | 120
52 | 5996 | 580 | 21·6 | 21·5 | 125
51 | 5919 | 430 | 29 | 28·5 | 122·5
50 | 5850 | 350 | 36 | 37 | 129·5
49 | 5783 | 275 | 45·5 | 47 | 129·2
48 | 5720 | 215 | 58 | 60 | 129
47 | 5658 | 170 | 73·4 | 76 | 129·2
46 | 5596 | 140 | 89·3 | 92 | 129
45 | 5538 | 125 | 100 | 98 | 122·5
44 | 5481 | 125 | 100 | 100 | 125
43 | 5427 | 130 | 96·1 | 97 | 126
42 | 5373 | 150 | 83 | 85 | 127·5
41 | 5321 | 180 | 69·4 | 65 | 117
40 | 5270 | 215 | 59 | 45 | 96·7
39 | 5221 | 250 | 50 | 30 | 75
38 | 5172 | 290 | 43 | 1·5 | 723·2
37 | 5128 | 335 | 37 | 16 | 53·6
36 | 5055 | 380 | 33 | 11·5 | 43·7
34 | 5002 | 500 | 25 | 7 | 35
32 | 4994 | 650 | 19 | 4 | 26
30 | 4848 | 850 | 14 | 2·5 | 23·3
28 | 4776 | 1100 | 11·4 | 2 | 22
26 | 4707 | 1500 | 8·3 | 1·5 | 22
24 | 4639 | 2000 | 6·2 | 1 | 20
22 | 4578 | 2700 | 4·6 | 5 | 13·5
18 | 4459 | 4750 | | |
14 | 4349 | 7500 | | |
10 | 4245 | 11000 | | |
--------+--------+-------------+------------+-----------+-----------

TABLE XII.--M.’S LUMINOSITY CURVE COMPARED WITH THE NORMAL (see Fig. 30).

-------+--------+--------+-------+----------+-----------+----------
I. | II. | III. | IV. | V. | VI. | VII.
-------+--------+--------+-------+----------+-----------+----------
Scale | Wave- | Mean | Mean | Normal |Difference |
number.| length.|reading.|reading|luminosity|of last two|Difference
| | |× 1·8. | curve, | columns. | × 5·15.
| | | |centre of | |
| | | | eye. | |
-------+--------+--------+-------+----------+-----------+----------
61 | 6839 | 2 | 3·6 | 4 | ·4 | 2·57
59 | 6621 | 7 | 12·6 | 12·5 | -·1 | ·51
57 | 6423 | 18 | 32·4 | 33 | +·6 | 3·09
55 | 6242 | 36 | 64·8 | 65 | ·2 | 1·03
53 | 6074 | 49 | 88·2 | 89·5 | 1·3 | 6·71
52 | 5996 | 52 | 95·4 | 96·5 | 1·1 | 5·66
51 | 5919 | 54 | 97·2 | 99·5 | 2·3 | 11·8
50 | 5850 | 54 | 97·2 | 100 | 2·8 | 14·4
49 | 5782 | 52·5 | 94·5 | 99·5 | 5·0 | 25·7
48 | 5720 | 50 | 90 | 97 | 7·0 | 36·0
47 | 5658 | 46 | 82·8 | 92·5 | 9·7 | 49·9
46 | 5596 | 41 | 73·8 | 87 | 13·2 | 68·0
44 | 5481 | 32 | 57·6 | 75 | 17·4 | 89
42 | 5373 | 23 | 43·2 | 62.5 | 19·3 | 99
40 | 5270 | 17 | 30·6 | 50 | 19·4 | 100
38 | 5172 | 10 | 17·5 | 35·5 | 18 | 93
36 | 5085 | 4 | 7·2 | 24 | 16·8 | 86·5
34 | 5002 | 1·0 | 1·8 | 14·5 | 12·7 | 65·5
31 | 4885 | ·5 | ·7 | 6·5 | 5·8 | 37·7
28 | 4776 | 0 | 0 | 4 | 4 | 20·6
-------+--------+--------+-------+----------+-----------+----------

TABLE XIII.--MISS W.’S CURVES (see Fig. 39).

--------+---------+-----------+--------------+------------
Scale | Wave- | Readings. | Extinction in| Persistency
number. | length. | | 1/100000. | curve.
--------+---------+-----------+--------------+------------
63 | 7082 | 0 | |
62 | 6957 | 1 | |
60 | 6728 | 7 | |
58 | 6520 | 18 | |
57 | 6423 | 28 | |
56 | 6330 | 43 | |
54 | 6152 | 76 | 900 | 2
52 | 5996 | 90 | 250 | 7
50 | 5850 | 95 | 130 | 13·5
48 | 5720 | 93 | 60 | 29
46 | 5596 | 83 | 34 | 51
44 | 5481 | 71 | 22 | 80
42 | 5321 | 58 | 18·5 | 92
40 | 5270 | 46 | 17·5 | 100
38 | 5172 | 32 | 18 | 94
36 | 5085 | 21 | 19·5 | 90
34 | 5002 | 12·5 | 22 | 79
32 | 4924 | 7 | 27 | 65
30 | 4848 | 4·5 | 34 | 51
28 | 4776 | 3·0 | 40 | 38·5
25 | 4675 | 1·5 | 60 | 29
20 | 4518 | 0·4 | 250 | 7
19 | 4488 | 0·0 | 350 | 5
16 | 4404 | -- | 600 |
--------+---------+-----------+--------------+------------

INDEX

PAGE

Absorption by the Yellow Spot 90

Artificial Spectrum 33

Cases of Defective Colour Vision unrecognised 67

Clerk Maxwell’s Colour-Box 42

Clerk Maxwell’s Colour Curves 47

Colour, and the Sensations required to produce it 50

Colour Blindness due to Disease 137

Colour-Blind Persons see a Grey in the Spectrum 65

Colour Discs 32

Colour Fields 13

Colour Matches made by the Colour Blind 70

Colour Patch Apparatus 18

Colour Patch Apparatus, Original Form of 19

Comparison of the Young and Hering Theory 189

Complex Colours matched by Simple Colours 22

Contrast Colours 187

Curious Case of Congenital Colour Blindness, A 164

Dalton Colour Blindness 58

Daltonism, or Colour Blindness 57

Defective Form Vision connected with Colour Deficiency due to
Disease 138

Definition at different parts of the Retina 11

Enfeebled Spectrum Luminosity 98

Exhibiting Colour Blindness by Colour Discs 74

Extinction and Persistency Curves of Green-Blind Persons 127

Extinction and Persistency Curves of Monochromatic Vision 125

Extinction and Persistency Curves of Red-Blind Persons 127

Extinction of Colour 105

Extinction of Light by the Centre and Periphery of the Eye 114

Extinction of Colour of equal Luminosity 110

Extinction of Light in the Spectrum 109

Eye: Explanation of its Functions 3

Fatigue of the Retina 6, 30

Field of View 10

Fovea Centralis 4

Fundamental Light 34

Green-Blind Person’s Description of the Spectrum, A 64

Green Monochromatic Vision 131

Helmholtz Diagram of Sensations 38

Heredity in Colour Blindness 58

Hering’s Colour Vision Theory 52

Hering’s Theory not tri-chromic 57

Holmgren’s Colour Tests 169

Kœnig’s Colour Sensation Curves 49

Lissajou’s Figures 37

Luminosity of the Spectrum to the Centre of the Eye, the Fovea
Centralis, and outside the Yellow Spot 88

Luminosity of the Spectrum to partially Colour Blind 86

Luminosity of the Spectrum to the Colour Blind 81

Luminosity of the Spectrum to the Normal Eyed 78

Malingerers, Detection of 185

Matching Colours by Mixtures of Simple Colours 26

Maxwell’s Colour Equations 202

Maxwell’s Curves for Red Blindness 69

Measurement of Colour Fields 207

Monochromatic Vision and the Spectrum 66

Number of Cones in the Eye 8

Optograms 9

Pellet Tests 146

Pendulum Experiments 36

Persistency Curves 119

Primary Colours 25

Primary Pigment Colours 27

Progressive Atrophy of the Optic Nerve 153

Purkinje’s Figures 7

Purples 24

Red and Green matched 72

Red-Blind Person’s Description of the Spectrum, A 63

Retina, Structure of 6

Retinal Fatigue 6, 30

Rods and Cones 8

Seat of Visual Sensation 7

Sensation Curves in Terms of Luminosity 93

Sensitiveness of the Eye 121

Simple Colours 17

Simulation of Red and Green Blindness 175

Spectrum described by the Tobacco Blind, The 143

Spectrum Test for Colour Blindness 181

Table of Wave-Lengths 17

Tables 211

Tobacco Ambyopia 140

Tobacco Blindness, Examples of 148

Violet Blindness 73

Visibility of an Object in light of different Colours 123

Visual Purple 9

White Monochromatic Vision 158

Wool Test, The 170

Yellow Spot 4

Yellow Spot and Colour Mixtures, The 28

Young’s Theory, Modification of 196

WORKS ON PHOTOGRAPHY

BY

CAPT. W. DE W. ABNEY, C.B., D.C.L., F.R.S., _Late Royal Engineers_.

INSTRUCTION IN PHOTOGRAPHY. Post 8vo., 3_s._ 6_d._

PHOTOGRAPHY WITH EMULSIONS. Post 8vo., 3_s._

NEGATIVE MAKING. Post 8vo., 1_s._

ART AND PRACTICE OF SILVER PRINTING. Written in conjunction with Mr.
H. P. ROBINSON. Post 8vo., 2_s._ 6_d._

PLATINOTYPE. Written in conjunction with LYONEL CLARKE. Post 8vo.,
2_s._ 6_d._

LONDON: SAMPSON LOW, MARSTON & COMPANY, LIMITED,
ST. DUNSTAN’S HOUSE, FETTER LANE, FLEET STREET, E.C.

LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
STAMFORD STREET AND CHARING CROSS.

Transcriber’s Notes

Punctuation, hyphenation, and spelling were made consistent when a predominant preference was found in the original book; otherwise they were not changed.

Simple typographical errors were corrected; unbalanced quotation marks were remedied when the change was obvious, and otherwise left unbalanced.

Illustrations in this eBook have been positioned between paragraphs.

The index was not checked for proper alphabetization or correct page references.