222 Mr. J. Z. Laurence on the Sensibility 



influence of light in any part of the experiment; that, in a word, 

 a very close sympathy exists in the two retinae, of which the 

 consentaneous action of the two irides is probably but a reflex 

 nervous consequence. 



I may here allude to a distinction in ocular spectra which has, 

 I believe, not been taken much account of by observers of these 

 phenomena. Some spectra seem as if projected on the plane to 

 which we direct the eye, and in that case appear, as I have found 

 from numerous measurements, linearly magnified in proportion 

 to the distance of the eye from the plane of projection. Other 

 spectra, on the contrary, are perceived, so to say, in the eyeball 

 itself, and are of a subjective nature. Independent of the dif- 

 ferences of their apparent seats, the two classes of spectra pre- 

 sent certain other well-defined distinctions. Projected spectra 

 are only perceived with the eyes open, and are generally but 

 faint in colour; while subjective ones maybe seen with the eyes 

 shut, and are always intense in colour. At the same time I am 

 disposed to ascribe the differences of colour, in a certain degree, 

 to the diluting influence of extraneous light; for projected 

 spectra are always seen more vivid in a dark room than in day- 

 light. 



The green spectrum observed on a sheet of white paper, after 

 prolonged contemplation of a red wafer, has been commonly 

 explained thus : — " When the eye has been for some time fixed 

 on the red wafer, the part of the retina occupied by the red 

 image is deadened by its continued action, and insensible to the 

 red rays which form part of the white light from the paper; 

 consequently will see the paper of that colour which arises from 

 all the rays in the white light of the paper, but the red; that is, 

 of a bluish-green colour, which is therefore the true complemen- 

 tary colour of the red wafer*." 



That this explanation is not correct seems to me to be proved 

 by the following experiment : — 



I, at night, made a room (which is provided with thick Ame- 

 rican-leather blinds for ophthalmoscopic purposes), to all appear- 

 ance, absolutely dark, then viewed with the left eye a small 

 aperture in a dark box covered with a piece of emerald-green 

 glass, behind which was the nearly white flame of a lamp. The 

 right eye was kept closed, and covered with a thick handkerchief. 

 After a time I blew out the light in the box, and looked at a 

 screen covered with a sheet of dead black paper. With the left 

 eye a large carmine-coloured projected spectrum of the flame 

 could be seen; with the right eye I generally perceived no spec- 

 trum at all, or if any, but of a very faint tint. But if the latter 

 eye was exposed to a white light during, the first part of the ex- 

 * Brewster's ' Optics/ 1831, p. 305. 



