On Vision. 27 



the Philosophical Magazine, except Mr, P,, that would confound 

 *' my idea" of the manner in which vi:sioii is acconii)ii>.hed, with 

 that of " Ins jriend," who seems to have understood iieaily as 

 much of op»« as himself. Much as it may surprise Mr. P., I can 

 assure him^ that my theory is verily huilt upon his " old opinion, 

 that ligJit acts upon the oplic 7ierve, and excites sensalion" — 

 there, and not upon the retina. 



In mv former communication (No.220,p. 1 17), I merely stated 

 the principle of my theory, and introduced an experiment which 

 I had made in support of that principle. Should any gentleman 

 scientijicaliy answer my arguments, and invalidate the exjjeri- 

 raents adduced in the pamphlet, where the theory is fully deve- 

 loped, I should at once renounce it as an untenahle hypothe!^is ; hut 

 till then I am warranted in retaining an opinion sanctioned by high 

 philosophical authority, that " the phcenomena are more philoso- 

 phically explained by this theory than any hitherto proposed." 



In the whole circle of science, no theory has obtained more 

 admiration than that of light and colours, as established by New- 

 ton upon his experiments with the prism. But it certainly owes 

 more to the mathematical genius of its author than to tlie truth 

 of its physical principles. He assumes it as a first principle, 

 supported by mathematical calculation, that the prismatic spec- 

 trum is an image of the sun ; and then proceeds, with the same 

 rigid adherence to mathematical demonstration, to show, that 

 the prism decomposes the rays that enter the foramen in the win- 

 dow-shutter, so as to produce a spectrum vt'liose length is about 

 five times its breadth, and composed of seven colours. This 

 altogether is taken as a demonstration that light is composed 

 of so many different kinds of rays, that differ in refrangibility in 

 the same order in which they stand to the rcfringcnt angle of the 

 prism. In the formation of the spectrum we are to suppose an 

 infinite number of circles, composed of rays diflerently refran- 

 gible, and projected a little beyond eacli other, so that the as- 

 seml.'lage of circles forms an oblong imago, the extremities only 

 of which are circidar. 



But the principle is fallacious, that supposes the prismatic s])ec- 

 trum to be an image of the sun, when it is evidently an image of 

 the hole in the wiudow-shutter, with its polari/ed light. As for 

 the lengthening of the spectrum, there is no proof that the rays 

 arc decomposed and separated i)V the prism. The fact is, that 

 the pri>>m modifies the direction of the rays coming from the 

 aperture, which otherwise would be dispersed; so thiit, so far is 

 the piism from decomposing the rays, it collects and transmits 

 them in such directions as to mix and compound them in the 

 spectrum. Having made these remarks, I shall now endeavour 

 to demonstrate how the colours in the prismatic spectrum are 



produced. 



