Part 4
15. _The dichromic distinguish between the colours of the normal-sighted, which are included in one of theirs by their relative luminosity and the difference of saturation which is apparent to them._ A test should therefore have the means of presenting colours of different saturation in succession.
16. _Colour-blindness is frequently associated with very high intelligence and exceptional ability._
V. _The Lantern Test._[11]--1. _Description of apparatus._ The lantern contains four discs: three carrying seven coloured glasses, and one with seven modifying glasses. Each disc has a clear aperture. The other mechanical details are: an electric or oil lamp with projecting accessories, a diaphragm for diminishing the size of the light projected, handles for moving the discs and the indicator showing the colour or modifier in use. The diaphragm is graduated in respect to three apertures to represent a 5-1/2-inch railway signal bullseye at 600, 800, and 1000 yards respectively when the test is made at 20 ft. The glasses are as follows:---
_Coloured glasses._
1. Red (A and B). 2. Yellow. 3. Green. 4. Signal Green. 5. Blue. 6. Purple.
_Modifying glasses._
7. Ground glass. 8. Ribbed glass. 9. Neutral (No. I). 10. " ( " II). 11. " ( " III). 12. " ( " IV). 13. " ( " V).
[11] Made by Reiner and Keeler, 9, Vere Street, W.; and Meyrowitz, 1a, Old Bond Street, W.
[Illustration: Fig. 3.]
It will be noticed that three of the discs are similar in every respect. In some of my lanterns the two reds are put at the end of the series of colours and numbered Red 1 and Red 2. This makes no important difference, but the arrangement given here is more convenient. It should be noted that Red 1 corresponds to Red B and Red 2 to Red A. If the electric lamp should get broken the projecting apparatus can be removed and an ordinary kerosene lamp placed behind the aperture.
[Illustration: Fig. 4.]
2. _Reasons for special construction._--The lantern has been constructed conformably with the requirements and facts of colour-blindness. All the facts I have given have been considered in constructing the lantern.
The examiner, on possessing a lantern for the first time, should carefully test himself with it and ascertain how the different lights appear to him with different conditions of general illumination. It is probable that certain improvements may suggest themselves to him, therefore, I think it will be advisable to deal with certain of these points, as it will help the examiner in the use of the lantern. The colours have never been altered, and I certainly should have altered them if I could have improved the lantern by doing so. I have never met a single colour-blind person who has not been readily and easily detected with my lantern, though I have examined many who have passed other lanterns and in some cases a number of other tests for colour-blindness. In most cases one turn of the wheel will be sufficient to make a colour-blind person disclose his defect.
The examiner may be dissatisfied with the colour of the blue; let us, therefore, compare an examination of a normal-sighted person with that of an ordinary dichromic. The normal-sighted person will name every one of the colours with ease and certainty, with perhaps the exception of the blue, with which he is in some doubt. Here is the result of an examination of an ordinary dichromic: he called the yellow, green; the green, red; the signal green, no colour; the blue, blue; the purple, green; red A, no colour or light; red B, green. It will be noticed that the only colour that he has correctly named is the blue. We can try him again and again, and though he will mistake all the other colours he will always name the blue correctly. The examiner will have learnt from this several important facts. He will see that the colour-blind are really guided by their sensations of colour, and that it is not simply a matter of guessing. The more an examiner has practical experience of colour-blindness, the more will he recognise the fact that the colour-blind are guided by their sensations of colour. He will notice that the dichromic has readily recognised the blue which was scarcely apparent to him (the examiner), and therefore cannot have overlooked as a matter of carelessness colours which are much more apparent to the normal-sighted. The blue is a valuable colour for other reasons, for though it is not a colour on which I reject candidates, anyone miscalling it must be very carefully examined before he is passed. The trichromic generally call this blue, green. If we wish to obtain a purer blue, we can do so by combining the blue or the purple with the signal green.
Again, the examiner may think that it might be better to have an apparatus which showed two or three lights instead of only one. I will therefore give my reasons for adopting only one. This point was one which occupied my attention for a considerable time, especially in view of the fact--which, as far as I am aware, I was the first to discover--that simultaneous contrast is increased to the colour-blind. I was naturally anxious to turn this fact to account. I found, however, that I gained nothing by increasing the number of lights, and that in many cases it was a source of error. A second or third light could have been easily added to my lantern, but besides unnecessarily complicating the apparatus it would have served no useful purpose. All the results which are obtained with simultaneous contrast are obtained even more effectively with successive contrast. It will be noticed that when lights are seen in ordinary conditions they are conditions of successive contrast, and not of simultaneous contrast. An observer rarely keeps his eyes definitely fixed on one light whilst he names those adjacent to it, but moves his eyes so that the images of the respective lights fall successively on his foveas. When more than one light is employed, all the disadvantages of matching as against naming are introduced. It will be seen that by presenting one light after another we are fulfilling all the necessary conditions, only that the light is moved instead of the eyes, sufficient time being allowed to elapse to enable a normal-sighted person to readily recognise the true colour of the next light without being confused by the after-image of the one he has just seen. Many nervous normal-sighted would name a yellow light seen between two red lights as green, and it does look green to them from ordinary physiological conditions. They look first at one red light, then immediately at the yellow light, then at the second red light, and then again at the yellow light until they feel sure that the centre light is a green light, and say so. I have never met with a normal-sighted person who has miscalled the unmodified light of my lantern, either red or green. Many humble, nervous normal-sighted persons are under the impression that they are colour-blind, and yet would make perfectly efficient officers. Many of these men have been told by their wives or other persons that they are colour-blind, and, believing this, try to see colours which are not visible to them. I have examined many persons of this description, and have noted the ease and accuracy with which they have gone through the tests for colour-blindness when they have been assured by me that they were normal-sighted. On the other hand, it is often very difficult to convince a self-reliant, colour-blind person that he is colour-blind. He is on the look-out for the small differences which he notices between colours, and the fact of having another light for comparison gives him the desired clue, and, though colour-blind, he passes the test.
The material is the best possible, as it will not fade like all dyed substances, and therefore all records made with one set of apparatus will be uniform. Again, a coloured light has none of the accessory qualities which enable the colour-blind to pass through other tests. Thus many dichromics will call the yellow glass red or green, who would not think of putting a yellow with a green or red wool, on account of the difference in luminosity. He will, in the same way, if told to pick out colours in the Classification Test to match the colour of the light shown, have to depend upon his colour-perception. This is a useful method with nervous and undecided candidates. The objection to it is that it cannot be carried out in the dark or in a dark room. The Test is not open to any of the objections which may be urged against the method of simply naming colours, because the character and intensity of the colour may be changed at will.
The method is better than that of direct comparison, because the candidate is forced to use his colour-perception, and has to compare the colour seen with previous impressions of colour in his mind. By the use of neutral glasses, etc., I have obviated the fallacy of the method of naming colours (namely, that these can be distinguished by their saturation and luminosity), and forced the individual to depend upon his colour-perception, and not upon some other accessory quality of the object seen.
No amount of coaching will enable a colour-blind person to pass this test, whilst almost any other may be passed in this way. I have tried on many occasions to coach a man so as to pass my lantern, and without success. The combinations are so numerous that the only result is to make the colour-blind man nervous and doubtful and more easily detected than before. This has occurred with men who could pass other tests with ease.
The test also has a quality possessed by no other--namely, that of enabling the examiner to reject dangerous persons and dangerous persons only, the lower degrees of colour-blindness being allowed to pass.
3. _Special directions for conducting the test._--(1) The candidate should be seated at a distance of twenty feet from the lantern. (2) He should be asked to name the colour of the light produced by a coloured glass (1 to 6) alone, or in combination with another coloured glass or glasses, or with the modifying glasses (7 to 13). (3) A candidate should be rejected (i) if he call the red, green, or the green, red, in any circumstances; (ii) if he call the white light, in any circumstances, red or green, or vice versa; (iii) if he call the red, green, or white lights, black, in any circumstances. (4) A candidate who makes mistakes, other than those mentioned above, should be put through a very searching examination. It is not necessary to have the room absolutely dark; in fact, I prefer a certain amount of light. The examiner can, if he wish, make the test at night in the open air.
The examiner should on no account conduct the examination on any regular plan, because the candidate, anxious to pass, finds out from persons who have already passed the order and method of the examination, and so, though colour-blind, might obtain a certificate. Any one of the glasses may be shown first, and the candidate required to name the colour of the light. The following will serve as an example of the method to be employed in testing a candidate. A red being shown, the candidate is required to name its colour. Then a blue or green may be substituted. It is best to use the largest aperture at first and to show all the colours on one disc. This will give confidence to the normal-sighted candidate, whilst most of the colour-blind will be detected. In the case of candidates who appear to be normal-sighted and yet very nervous, there is no harm in telling them after they have named all the colours on the disc correctly that this is the case. No comment should, however, be made on individual answers. Then one of the neutral, ground, or ribbed glasses should be inserted, not the slightest intimation being given to the candidate of the nature of the colour. He should be asked to name or describe the light, and the answer, if incorrect, together with his other replies, carefully recorded. The other glasses may then be shown, a combination of the neutral, ground, ribbed, and coloured glasses being used at irregular intervals.
When the candidate has been examined with the largest aperture, the examiner can go through the same procedure with one of the smaller apertures. I have found the third aperture the one which is most generally useful. On account of the great diminution of total luminosity caused by the diminished area of the light source, the three smallest apertures can only be used in a dark room.
If a candidate hesitate about a colour and ultimately name it correctly, a second and, if necessary, a third glass of the same colour should be combined with the first. The fact that in one case a single glass is used, and in another two or three of the same coloured glass, makes very little difference in the colour of the light to the normal-sighted. This is not the case with the colour-blind; a dichromic who has hesitated about a green and then correctly named it may emphatically call the light red when another green glass is put in front of the first.
Care must be taken when the candidate is going to be examined with two glasses at once, such as one of the neutral, ground, or ribbed glasses, and a coloured glass, that he does not see the light until both are in position, or else he may see the colour before it is modified in the necessary way.
If the candidate call the standard red, green; or the standard green, red, in any circumstances--that is, either alone or in combination with the modifying glasses--he is to be rejected.
The examiner should ascertain for himself how far the various colours are visible when modified with the neutral glasses. If the red and green be not visible with the thickest neutral in the conditions of luminosity and external lighting which the examiner is employing, he should use the darkest neutral which allows the colours to be plainly visible to the normal-sighted. In all cases of doubt the examinee should be asked to walk towards the lantern and told to say when the light is visible, and asked to name its colour. The distance at which the light is visible, and then that at which the colour is visible, should be noted and compared with the normal.
Particular attention should be paid to the answers given to the combination of the thickest neutral glass with the standard red and green respectively.
The examiner should utilise the fact that successive contrast is increased in the colour-blind, as this is an easy method of detecting the trichromics. The red having been shown, the light should be quickly changed to yellow or clear, the examiner’s hand being placed over the aperture if there be any intervening colours. It is necessary that the yellow should be shown immediately after the red without any intervening colours being first seen by the candidate. The normal-sighted do not see any change in the yellow or clear when they are shown after the red light, but the trichromic call the yellow light, green. The examinee should then be shown the green light, and then the yellow or clear, in the same way as mentioned for the red. The normal-sighted will easily recognise the yellow, but the trichromic will call it red. This portion of the examination must never be omitted in any examination in which the candidate is passed. The two divisions of the test--that is, showing yellow immediately after red and after green--may be used at different periods of the examination, and, if there be any doubt, repeated.
An examiner should, as far as possible, with the exceptions given in the instructions, avoid all conversation with the candidate, simply asking, “What colour is this?” and recording the answer without comment. If an examiner after each answer say, “Quite right,” or some such expression, the following is likely to occur. The candidate after, say, six correct answers, makes a mistake; the examiner says, “Are you sure?” Then the candidate knows at once that he has made a mistake, and makes a guess, very probably a correct one. When a similar colour is shown subsequently, he remembers the mistake he made, and gives the second, and probably the correct answer.
In addition to being an efficient test, it is a very rapid test, as many men who have been certified as normal after a lengthy examination with other tests have at once disclosed their defect by calling the green light of the lantern red. Many are under the impression that in an examination with the lantern the dichromics simply guess. This is entirely wrong. A man who did guess would know that he was incompetent. I find that men have named the coloured lights in strict accordance with their colour-perception. A man may, however, guess if examined by an inexperienced and ignorant examiner, who when the examinee has made a mistake promptly corrects it in a cross tone. A normal-sighted person will guess when examined in this way. The examiner must receive the examinee with a smiling face and courteous manner, and appear pleased and satisfied with the answers, no matter what they may be. The candidate is then placed at his ease, and answers according to his colour-perception. It will be noticed that the lantern detects those who have a slightly diminished colour-perception, as well as the dangerous varieties of colour-blindness. The former undoubtedly are not as efficient as those who have a normal colour-perception, so that a definite standard will have to be fixed, as in the case of visual acuity. Further details will be found in my book on Colour-Blindness.[12]
[12] _International Scientific Series._ Kegan Paul & Co., 1909.
_Summary of method of examination._--(1) Show all the colours on one disc with the largest aperture. (2) Show the reds, greens, and yellow modified by the neutral glasses. (3) Show all the colours on one disc with Number 3 aperture. (4) Show red, then immediately afterwards yellow with largest aperture. Then show green and yellow immediately afterwards. (5) Test the candidate with the red, green, and yellow with the smallest aperture. (6) Show the neutrals or ground glass alone. (7) Show blue made by combining blue or purple with the signal green. (8) Show a colour, for instance, green, and then combine another glass of the same colour. (9) Show the red produced by the combination of purple with red A. (10) Give the combination of red A and signal green.
VI. _Other tests for colour-blindness._--I have three other tests for colour-blindness: the Classification Test, the Pocket Test, and the Colour-perception Spectrometer. I have also devised an instrument for estimating the exact amount of red, at different wave-lengths, which is necessary to neutralise the complementary in different persons.
1. _The Classification Test._--(_a_) _Description._--This test consists of 4 test colours and 180 confusion colours; 150 coloured wools, 10 skeins of silk, 10 small squares of coloured cardboard, and 10 small squares of coloured glass. The whole series of colours is represented. In addition, there are a large number of colours which have been chosen by colour-blind persons as matching the test colours. The test colours are Orange, Violet, Red, and Blue-green, labelled I, II, III, and IV respectively. The colours are chosen with the view of presenting as much difficulty as possible to the colour-blind, and as little as possible to the normal-sighted. The colour-blind find especial difficulty in matching or naming a colour lying at the junction of two of their colours. As the normal-sighted often find difficulty in saying which colour predominates in a blue-green, so do the tetrachromic with their purple-green, or the trichromic with their red-green. A colour-blind person may, however, match a colour correctly which corresponds to the centre of one of his colours. In addition to choosing those colours for tests which are particularly liable to be mistaken for other colours by the colour-blind, I have used coloured materials of different kinds--wools, silks, glass, and cards--so as to force the colour-blind to judge by colour, and not by saturation or luminosity. (See Fig. 5.)
[Illustration: Fig. 5.]
(_b_) _Method of examination._--The candidate should be given the four test colours, and, having named each, he should be told to select all those which are similar in colour to the test colour. He should be told to pay no attention to the fact of a colour being lighter or darker; as long as it is the same colour it should be put with the test skein. The examiner should not go through the test before the candidate first of all, neither should one candidate be allowed to watch another making his selection. A shrewd colour-blind person might pass the test if he had seen a normal-sighted person go through it previously. In order to show the candidate the difference between a shade and a colour, the examiner should take one of the wools which is not a test colour--blue, for instance--and pick out four or five shades of the colour. The wools should be arranged without the knowledge of the candidate, so that a yellow or a grey is placed beside a red and the examinee asked to name its colour. At another period of the examination the yellow should be placed adjacent to a green, and the examinee again asked to name it.
The examinee may pick out a certain number of colours correctly, and then stop, saying that there are no more exactly like the test colour. This may embarrass the examiner; he should, however, examine any candidate who has omitted any colours as carefully as if mistakes had been made. He should ask the candidate to match one of the omitted colours.
The examiner will soon find out from experience those colours which are named and matched wrongly by colour-blind persons; he should ask the examinee to name some of these colours.
Any candidate should be rejected who calls an orange or red, green or brown; black, red or vice versa; or green, either purple, rose, red, grey, brown, or violet. Similar mistakes in matching necessitate objection. A candidate who puts purple, rose, or blue with violet, or yellow-brown with orange is most probably dangerously colour-blind and should be very carefully examined. There are cases which pass the Holmgren test with ease that fail in the most conclusive manner with my Classification Test. They put green with orange, brown and black with red, and grey with blue-green. This is due to a different selection both of test colours and confusion colours. Orange is by far the most important test colour, and its confusion with green by the dichromics is very conclusive. The three other test colours, violet, red, and blue-green, represent both ends of the spectrum and the neutral point in dichromic cases, and practically these colours are those with which most mistakes are made. This test can only be regarded as supplementary to the Lantern Test.