540 



NA TURE 



[Al'RIL 6, 1893 



had any prejudicial effect, and therefore this extinction 

 curve may be taken as correct. 



Fig. 3. — Extinction of the Spectrum. 



In the curves there are two branches at the violet side, 

 and this requires explanation. One shows the extinction 

 when viewed by the most sensitive 

 part of the eye, wherever that may 

 be, and the other when the central 

 portion of the eye was employed. 

 The explanation of this difference in 

 .perception is chiefly as follows : — 



In the eye we have a defect — at 

 least we are apt to call it a defect, 

 though no doubt Providence has made 

 it for a purpose — in that there is a 

 yellow spot which occupies some 6° 

 to 8° of the very centre of the retina, 

 and as it is on this central part that 

 we receive any small image, it has a 

 very important bearing on all colour 

 experiments. The yellow spot absorbs 

 the blue-green, blue, and violet rays, 

 and exercises its strongest absorption 

 towards the centre, though probably 

 absent in the very centre, that is, in 

 the " fovea centralis," and is less at 

 the outer edges. That absorption of 

 colour by the yellow spot takes place 

 can be shown you in this way. Any 

 colour in nature can be imitated by 

 mixing a red, a green, and violet 

 together, and with these I will make 

 a match with white and then with 

 brown, two very representative colours, 

 if we may call them colours. Now if 

 I, standing at this lecture table, 

 by mixing these three colours ' together, using 

 NO. 1223. VOL. 47] 



a large patch, the image will fall on a part of 

 the retina of considerably larger area than the 

 yellow spot, and it will appear too green for those at a 

 distance ; but it is correct for myself. If I place a mirror 

 at a distance, and make a match again by the reflected 

 image, the match is complete for us all, as we all see it 

 through the yellow absorbing medium. If I look at it 

 direct from where I stand the match is much too 

 pink. It may be asked why the comparison patches and 

 the mixed colours do not always match since both 

 images are received on the same part of the retina. The 

 reason is that the green I have selected for mixture is in 

 the part of the spectrum where great absorption takes 

 place, whilst the comparison white contains the green 

 of the whole spectrum, some parts of which are much 

 less absorbed than others. I may remark that just out- 

 side the yellow spot the eye is less sensitive to the red 

 than is the centre, and this is one additional cause of 

 the difference. See Fig. 5. 



More on this subject I have not time to say on this 

 occasion, but it will be seen that the extinction of light 

 for the centre and the outside of the eye differs on account 

 of this. 



I must take you to a theory of colour vision which, 



I though it may not be explanatory of everything, at all 



events explains most phenomena — that is, the Young- 



I Helmholtz theory. The idea embodied in it is that we 



I have three sensations stimulated in the eye, and that 



1 these three sensations give an impression of a red, a 



j green, and a violet. These three colours I have said can 



; be mixed to match any other colour, or, in other words, 



j the three sensations are excited in different degrees, in 



I order to produce the sensation of the intermediate spec- 



I trum colours, and those of nature as well. 



j The diagram Fig. 4 shows the three sensations as 



i derived from colour equations made by Koenig. It will 



be seen that there are three complete colour sensations, 



all of which are present in the normal eye. I would ask 



j you to note that at each end of the spectrum only one 



sensation is present, viz. at the red end of the spectrum, 



the red sensation, and at the violet end the violet. 



match a white 



Fig. 4. — Colour Sensations. 



This is a matter of some importance, as we shall now 

 see. 



