COLOUR VISION. 279 



the line F, which are of a greenish-blue colour. Some of us have this spot 

 strongly developed. My own observations of the spectrum near the line F are 

 of very little value on this account. I am indebted to Professor Stokes for 

 the knowledge of a method by which anyone may see whether he has this 

 yellow spot. It consists in looking at a white object through a solution of 

 chloride of chromium, or at a screen on which light which has passed through 

 this solution is thrown [exhibited]. This light is a mixture of red light with 

 the light which is so strongly absorbed by the yellow spot. When it falls on 

 the ordinary surface of the retina it is of a neutral tint, but when it falls on 

 the yellow spot only the red light reaches the optic nerve, and we see a red 

 spot floating like a rosy cloud over the illuminated field. 



Very few persons are unable to detect the yellow spot in this way. The 

 observer K, whose colour equations have been used in preparing the chart of 

 the spectrum, is one of the very few who do not see everything as if through 

 yellow spectacles. As for myself, the position of white light in the chart of 

 the spectrum is on the yellow side of true white even when I use the outer 

 parts of the retina ; but as soon as I look direct at it, it becomes much 

 yellower, as is shewn by the point WC. It is a curious fact that we do not 

 see this yellow spot on every occasion, and that we do not think white objects 

 yellow. But if we wear spectacles of any colour for some time, or if we live 

 in a room lighted by windows all of one colour, we soon come to recognize 

 white paper as white. This shews that it is only when some alteration takes 

 place in our sensations, that we are conscious of their quality. 



There are several interesting facts about the colour sensation which I can 

 only mention briefly. One is that the extreme parts of the retina are nearly 

 insensible to red. If you hold a red flower and a blue flower in your hand 

 as far back as you can see your hand, you will lose sight of the red flower, 

 while you still see the blue one. Another is, that when the light is diminished 

 red objects become darkened more in proportion than blue ones. The third is, 

 that a kind of colour blindness in which blue is the absent sensation can be 

 produced artificially by taking doses of Santonine. This kind of colour blindness 

 is described by Dr Edmund Rose, of Berlin. It is only temporary, and does 

 not appear to be followed by any more serious consequences than headaches. 

 I must ask your pardon for not having undergone a course of this medicine, 

 even for the sake of becoming able to give you information at first hand about 

 colour blindness. 



