VOL. LXXIX.] PHILOSOPHICAL TRANSACTIONS. 5Q5 



XXI. u4n Attempt to explain a Difficulty in the Theory of Vision, depending on 



the Different Rejrangibility of Light, By the Rev. Nevil Maskelyne, D. £>., 



F. R. S., ^c. p. 256. 



The ideas of sight are so striking and beautiful, that we are apt to consider 

 them as perfectly distinct. The celebrated Euler, taking this for granted, has 

 supposed, in the Memoirs of the Royal Academy of Sciences at Berlin for 

 1747, that the several humors of the human eye were contrived in such a 

 manner as to prevent the latitude of focus arising from the different refrangi- 

 bility of light, and considers this as a new reason for admiring the structure of 

 the eye ; for that a single transparent medium, of a proper figure, would have 

 been sufficient to represent images of outward objects in an imperfect manner ; 

 but, to make the organ of sight absolutely complete, it was necessary it should 

 be composed of several transparent mediums, properly figured, and fitted toge- 

 ther agreeable to the rules of the sublimest geometry, in order to obviate the 

 eflfect of the different refrangibility of light in disturbing the distinctness of the 

 image ; and hence he concludes, that it is possible to dispose 4 refracting sur- 

 faces in such a manner as to bring all sorts of rays to one focus, at whatever dis- 

 tance the object be placed. He then assumes a certain hypothesis of refraction 

 of the differently refrangible rays, and builds on it an ingenious theory of an 

 achromatic object-glass, composed of 2 meniscus glasses with water between 

 them, with the help of an analytical calculation, simple and elegant, as his 

 usually are. 



He has not however demonstrated the necessary existence of his hypothesis, 

 his arguments for which are more metaphysical than geometrical ; and as it was 

 founded on no experiment, so those made since have shown its fallacy, and that 

 it does not obtain in nature. Also, which is rather extraordinary, it does not 

 account, according to his own ideas, for the very phenomenon which first sug- 

 gested it to him, namely, the great distinctness of the human vision, as was 

 observed to Dr. M. many years ago, by the late Mr. John Dollond, f. r. s. to 

 whom we are so much obliged for the invention of the achromatic telescope ; for 

 the refractions at the several humors of the eye being all made one way, the co- 

 lours produced by the first refraction will be increased at the 2 subsequent ones, 

 instead of being corrected, whether we make use of Newton's or Euler's law of 

 refraction of the differently refrangible rays. 



Thus Euler produced an hypothetical principle, neither fit for rendering a 

 telescope achromatic, nor to account for the distinctness of the human vision ; 

 and the difficulty of reconciling that distinctness with the principle of the 

 different refrangibility of light, discovered by Sir Isaac Newton, remains in its 

 full force. In order to go to the bottom of this difficulty, as the best probable 

 means of obviating it. Dr. M. calculated the refractions of the mean, most, 



4G2 



