Colour vision

by Sir William de Wiveleslie Abney

CHAPTER XIV.

Holmgren’s test, although a qualitative one, is most accurate in allowing a diagnosis to be formed, but it sometimes happens that a candidate is not satisfied that he has failed in passing the test, and wishes for another examination. Such a re-examination is best carried out by the spectrum method, which I will now describe.

The test with the spectrum is a very decisive one, and can be carried out with the patch apparatus ([Fig. 3]), [page 19]. Personally, I like to have some idea of the kind of colour blindness, if any, which exists by first using the Holmgren test. Should these tests show that a candidate is colour blind in any degree, a very excellent beginning is to try and find his neutral point in the spectrum—if he has one. To arrive at it we place two patches of light on the screen, one of colour and the other of white, the rotating sectors being in the last-named beam, and ask him to say when the two colours appear alike. It must be remembered that white is coloured from the effect of contrast as long as the colour alongside differs from it. A good point de depart is with the slit in the yellow, then to move it into the red, and then gradually to push it into the green. When here, if colour blind, he will say, “The two patches are nearly alike, but that the white is rather pink or green,” as the slit gets further towards the blue. The operator, whilst changing the colour, alters the sectors so that the luminosities are about the same. A point will be reached when the colour blind will say, “Now they are both alike, but one is rather darker than the other.” The sectors are altered until he says they are both alike, and the observation is satisfactory when he declares the two patches of light are both alike in colour and in darkness. It is curious how misleading the word brightness is to some people who are uneducated. I find it much safer to ask which is the darker colour, rather than which is the brighter. A little patience will always enable you to get a good observation. The place in the spectrum which is the neutral point is now noted. The neutral point is again found, but this time commencing in the blue. The same procedure is adopted as before, and we thus get a second reading for it, and the two will be found to be very close to one another. In difficult cases, four or five observations may be made, and the mean taken as a close approximation. So far the spectrum test has not shown whether the observer is red- or green-blind, except by comparing the position of the neutral point with that usually found by the two types. We have, however, an unerring criterion by the luminosity method. The red is placed beside the white, and he is asked to say which he considers the darker; he will give an answer of some kind, and probably protest that the two colours are not alike. A soothing answer will disarm his objection, and he will quickly see what you mean. If he be red-blind he will match in brightness a brilliant red and a feeble white; if he be green-blind he will make a match very similar to normal vision. In the case of the red-blind the slit is then moved into the extreme red, when he will say he sees but one patch of light, whilst the green-blind will see it as a person of normal vision would do. If time permits, the whole luminosity curve may be taken and registered. This is not essential, but interesting for reference. Where complete colour blindness exists, it should be possible to cause him to match a green with a red. To do this a second instrument, as described in [page 18], may be used, but it is quite sufficient if a piece of red glass, such as is used for railway signals, or of bottle-green glass, be placed in the white beam. There is then a red or green patch alongside the patch of spectrum colour. The red will stimulate the red sensation of the green-blind, but not being spectrum red it contains a certain amount of yellow, which stimulates the green sensation if the observer be red-blind. The green is of such a colour that it will stimulate both the red and the green sensations. In the path of the reflected beam between G and the prisms ([Fig. 3]) a sheet of plain glass is inserted, which reflects a proportion of white on to the red patch. The sectors are placed in this beam. If the red glass is being used, the slit is moved into the green near E, and the colour blind will say that both are the same colour, but one darker than the other. By opening or closing the slit in the spectrum, he will possibly say that both colours are alike and of the same darkness, but he may say one is paler than the other, in which case the white light must be increased or diminished by means of the sectors till equality of tone is established. This applies to the red-blind and the green-blind. The former will require a very bright red to match a feeble green, whilst with the latter the red will require a fairly light green. When the green glass is used the spectrum colour patch should be red, and the match be made as before. With the violet-blind the neutral point will be in the yellow, and with monochromatic vision matches can be made throughout the spectrum. So far it will be seen that no mention of any colour is required. It may next be advisable to ask him the names of colours. This is best done by placing the white patch of light over the spectrum colour patch, and opening and closing, as may be required, the sectors. If the sectors are closed it is very probable that correct guesses may be made, for then the colours will be saturated, and the colour blind, if they are intelligent, will know that a green to them is white or pale in colour compared with red, though of the same hue. If white be mixed with the red the wrong name is bound to be given, for they will be unable to distinguish it from the green, because it is then a less saturated colour. Passing from green to red and mixing the colour more or less with white, the most—I was going to say grotesque—telling mistakes are made. A further excellent test is to place a cell containing a solution of bichromate in the path of the reflected beam, and cause the observer to match its colour with the light coming through two slits, one in the red near C, and the other in the green near E. Defective colour perception will be well demonstrated. There are various other artifices which can be employed in the spectrum test, which would be too long to recount here, and if there be two sets of apparatus the tests are practically unlimited in number.

There are cases in which an observer who may have normal vision may wish to be reported as colour blind. A seaman’s life is not always a happy one, and a boy on a training-ship, knowing that a failure in colour vision will free him from a sea life, may be anxious to be told he has failed in colour vision. By “coaching” in the Holmgren test he might manage to obtain a “failure,” but a malingerer is sure to be detected by the spectrum method of testing. He may call diluted red green, and he may declare he sees a neutral point in the spectrum, but if he be tested with the diluted colours near his supposed neutral point, he is sure to fall into a trap. He will make a mistake in calling a patch green when it ought to be white, or white when it ought to be green, if he were truly colour deficient—indeed, a malingerer has no chance of escaping detection with the spectrum tests. It is not an uninteresting experiment to get an acute observer who has normal colour vision, and is accustomed to the spectrum test, to feign colour blindness, and examine him in this manner. He never fails to make such mistakes as would lead to his detection.

With the partially colour blind the same procedure may be adopted. In examination by the Holmgren wool test, slight mistakes will be made in matching the first two test-skeins. With the spectrum test the red will require a greater dilution with white before it will be matched with a green, even if it can be matched at all. Measures of the luminosity at four or five positions in the spectrum, extending from near the extreme red to the blue, will give an unerring criterion of the kind and extent of colour blindness from which they are suffering. The existence of a neutral point in the spectrum is sufficient to indicate that their blindness is of a nature to be dangerous in certain occupations. To some it may be a difficulty how a neutral point can be found in such cases, since all sensations are more or less present. The reason, however, was explained on [page 96].