of the Eye to Colour. 221 



both eyes are directed to the sky, a similar spectrum will be ob- 

 served, projected, as it were, on the surface of the heavens, but 

 much darker. But if after a time each eye is alternately opened 

 and closed, a rose-coloured spectrum is seen with the left eye, a 

 pale green one with the right. These appearances are seen still 

 better if, instead of the sky, a white screen is used as the plane 

 of projection in the second part of the experiment. At first an 

 almost black circular disc is seen; this becomes lighter and 

 lighter, till it is finally succeeded in the left eye by a bright 

 rose-colour disc, surrounded by a violet border ; in the right eye 

 by an equally bright green with a rose border. These spectra 

 sometimes appear as if upon the surface of the screen, sometimes, 

 on the contrary, as if originating within the eyeball itself, and 

 indeed may be even seen with both eyes closed. To see the 

 above phenomena in all their intensity, a slightly different plan 

 must be adopted. As the field of projection, a sheet of dead 

 black paper in a dark room is to be used; the spectra then seen 

 with either eye are the same, and their colours most splendid, 

 both as regards brightness and tint. At first an emerald-green 

 disc appears, surrounded by a narrow carmine, or perhaps, more 

 accurately, magenta border; the magenta tint is then seen to 

 encroach more and more upon the green, till the whole disc is of 

 the former colour, surrounded by a bluish-violet border; this last, 

 in its turn, invades the magenta, till the final spectrum is of one 

 uniform indigo-violet colour. 



The above is the general sequence of colours which I, and 

 other persons whom I have asked to perform the experiment, 

 have observed; but these are liable to exceptions. Occasionally, 

 the librating spectrum observed at the end of the tube in the 

 first part of the experiment, acquires a faint rose, green, or violet 

 tint. Sometimes I have seen the spectra of the right and left 

 eyes, in the second part of the experiment, reversed as regards 

 colour. 



These facts appear to prove the following propositions : — 



1. That colour sensations may be excited in the retina, or 

 brain, altogether independently of any external colour-stimulus. 



2. That as an optical analysis of white light may be effected 

 by a prism, so with the eye we possess the power of effecting, 

 what may be called, its. physiological analysis. 



3. The last proposition tends to the conclusion that white light 

 consists of three fundamental colours — magenta, emerald-green, 

 and indigo-violet — corroborating in a remarkable manner the 

 opinions of Professor Maxwell and Dr. Young on the same 

 subject. 



4. That a colour sensation excited in one eye is generally felt 

 in the other, although this latter has not been exposed to the 



