Colour vision

by Sir William de Wiveleslie Abney

Fig. 35.

Left Eye.

Right Eye.

At [page 10], a description has been given of the manner in which the field for colour and light has been determined, and if this same method be pursued with persons suffering from this form of colour blindness we get some remarkable results. [Fig. 35] is the chart of the eye for red and for white, which was made by a case of tobacco blindness. The yellow spot is entirely affected, and, as is very common, it extends to the blind spot in the eye. At no place within that area can red be seen, though blue is immediately recognised. The extent of the field for white is that found under normal conditions, and except for the diseased area the same is true for the red. The fields for both eyes are given: that for the left eye in the left-hand chart, and that for the right eye in the right-hand chart. The small dark spots within the 5° area are places where the colour sensation is most defective. The part in the central dark area shaded with lines in this direction //// shows the portion of the field which is insensitive to red, though not to light, whilst the remainder of the shaded central area indicates the extent of the field which is sensitive to red. The field for light generally is also shown by the (approximately) rectangular unshaded area. Although the area occupied by the insensitive part of the retina is small compared with the whole, yet it is in that part which is used for distinct vision.

For testing for colour the apparatus, [Fig. 3], arranged so that the patch of colour has the white patch alongside, is the most useful, but it is as well then to use a surface of patch about ½ inch square only, and thus to confine the image as nearly as may be to the spot on the retina which is defective. These cases of central scotoma are by no means very easy to test; for it frequently happens that before they are able to distinguish that there are two patches side by side, they have to approach very close to the screen. If this be the case, however, it will usually be found that the patches of ½ inch side are still efficient, as the near approach of the eyes to the screen indicates a wide area as being affected, so that the image still lies within the diseased retinal area. In some instances the colours named will vary very considerably; sometimes, for instance, a red will be named as grey, and then immediately after as pale red. This is generally due to the diseased area being small, and a very slight change in the direction of the axis of the eye causes it to be seen in nearly its true colour, part being viewed with the diseased and part with the healthy portion of the retina. With the wool test, which we shall describe later, it is the commonest thing possible for colour-blind persons who have a central scotoma to match accurately the different test-skeins, for the reason that the images of the skeins of wool are so large that they are received on the parts of the retina which are not diseased. These same colours, however, if presented to them in small patches, will inevitably show the defect in vision.

With this end in view, I have had a set of brick-clay pellets some 3/16-inch in diameter, painted with water-colours mixed with soluble glass solution of the same colours as the wools. These are placed in a shallow tray, and presented to patients affected with this central colour blindness to pick out all the pellets which match reds and greens. They will tell you that they see neither one nor the other, though they will pick out the blue pellets unerringly. A red pellet they will match with a red, green, grey, or a brown one, and a green one with the same. If, however, you instruct them to direct their eyes a few degrees away from the tray, they will tell you they see all the colours, and as they endeavour to pick them out, they, with a natural instinct, direct their eyes again to the collection, when once more the colours vanish. It is almost piteous sometimes to see the distress which this simple test occasions. The sight of the colours for an instant and their immediate disappearance in the cases that I have tried, seem indicative of something terrible, for they usually have no idea of the cause of this (to them almost miraculous) phenomenon. I have seen these colour blind tested with a pair of ordinary bull’s-eye lanterns, placed side by side, with diaphragms of moderate size with coloured glasses, which can be changed at will, in front. At twelve feet distance they will often see both lights as one, but as they approach they will make out two lights and call them both white, or sometimes they will make a guess and call a green red, or vice versâ. It goes without saying that such eyesight is useless for reading signals, and indeed for any purpose whatever. Sometimes, but I believe this is rare, no colour whatever can be distinguished.

CHAPTER XII.

I will now give in full the result of the examination of a patient who was suffering from tobacco blindness. X., aged thirty-six, a commercial traveller, was suffering from rather severe tobacco amblyopia. The scotoma was a very marked one, and the loss of colour sensation most complete. Mr. Nettleship, who furnished the case, has kindly added the following remarks on the case:—

His acuteness of vision was 6/36 with the right eye and 6/60 with the left. He smoked half-an-ounce of “shag” daily and drank about four pints of beer. His sight had been failing for about two months. As is common in early stages of this disease the ophthalmoscope revealed no decided changes at the optic discs.