13Q ON Xnii LIMITS OF SINGLE TISION. 



this great geometiiciuii had no doubt it tuust be produce^l 

 bj' the various combination of forms und densities peculiar 

 to the humours of th^ eye* This theory, disputed at first 

 Dollond. by Dollond, sooiv found in that artist, convinced of hisj ef- 



rour, a man sufficiently skilful to reduce it tdtpractice; and 

 this union of genius and skill gave rise to those: Jjne instru- 

 ments, the invention of which must be classed with those 

 discoveries, that do most honour to the human iniud. A 

 comparison of the achromatic power of the human eye 

 with that of those instruments is the subject of these re- 

 flections. 

 Achromatic My experiments on the limits of single vision haying 



evr«omi)ared obliged me in some cases to give the ball of the eye a great 

 with that of decree of obliquity with respect to the luminous rays, which 

 iostruments. lavished to introduce into it, I could not avoid being asto- 

 nislied at the achromatic power of the humours, so well 

 adapted to prevent the decomposition of light, when so 

 many different causes seem as if they must necessarily pro- 

 duce it. The most perfect achromatic glasses, the objefct 

 glass of which should be of a focus sufficiently short to be 

 compared with the eye, one of an inch and a quarter for 

 example, would not bear an aperture of more than a fifth 

 of an inch, according to Euler's calculation. Allowing the 

 mean aperture of the pupil to be two lines [2| French], 

 the eye would be superior in achromatic power to the most 

 perfect glasses with the same aperture. But how can we 

 avoid being astonished at the consideration, that the size of 

 the aperture, which limits the achromatic power Of instrn- 

 ments of art, has no perceptible effect on that of the hu- 

 mours of the eye? and that the opening of the pupil may 

 be extended to three lines [3| Freiich] without altering the 

 clearness of the image, as 1 satisfied myself by dilating the 

 iris by applying an infusion of the leaves of deadly night- 

 shade so as to ex-pose the greater part of the anterior surface 

 of the crystalline ? 

 Farther proof A still further proof of the superiority of the achromatic 

 of it« superior- power of the eye over that of instruments is the application 

 of rays greatly inclmed to refractive surfaces. Experience 

 proves, that the best made object glasses, when greatly in- 

 clined to the rays of light, immediately exhibit signs of the 



decompoaitioii 



«y. 



