IN OPTICS. 115 



perceptible to sense, by the contrary refractions 

 which they undergo, from passing, successively, 

 through the different parts of the object-glass. 

 Hence, some have imagined, that, since objects, 

 in ordinary vision, are seen without colour, as 

 far as this depends on the refractions of the 

 eye, nature has furnished us with an instru- 

 ment, constituted upon principles similar to 

 those of the object-glass of an achromatic tele- 

 scope. But every one, the least acquainted 

 with the structure of the eye, must know, that 

 this cannot be the case, as the refractions in it 

 are all made one way*. And there are experi- 

 mental proofs, that compounded light is always 

 separated into its parts, by passing through the 

 eye. For if we interpose any opake substance 

 between us and a luminous body, so that only 

 a very small portion of this may remain visible, 

 it will appear to consist of three differently 

 coloured parts, red, yellow, and blue* The 

 reason, therefore, of objects being, for the most 

 part, seen colourless, must be elsewhere sought f. 

 Now let us suppose, that a luminous point is 

 the only object which is seen at any one time ; 

 should the focus of its mean refrangible rays be 



* There are indeed some exceptions to this, but not of 

 sufficient consequence to affect the present argument. 



f Dr. Maskelyne has very learnedly treated this subject ii* 

 the Philosophical Transactions, vol. Ixxix. part 2. 



