132 **^ "^^^ LIMITS OF SINGLE VISION. 



12 or 18 lines, it is true exhibited some slight appearances 

 of the decomposition of light; but these appearances, be- 

 ing but httle perceptible, and in many instances doubtful, 

 but tViese no I conceive are but little calculated to vieaken the supposi- 

 pioof, as ibey ^\q^ ^f ji,g pcfectiou of the achromatic system of the eve. 

 may arise from , , ,. , , , , ^ , , 



other causes In the first place, because these n-ises may be confounded 



than the defect with the penumbras that accompany all small bodies seen 

 '^^ ■ indistinctly, or with the shadows themselves which are co- 

 loured in certain circumstances: but more especially be* 

 cause the faint irises observed may be produced, not by tlie 

 unequally refracting power of the hunjours of the eye with 

 lespect to the various rays, but by the attraction of these 

 bodies for the luminous rays, which, being reflected by the 

 planes on which they infringe, graze their surface before 

 they come to the eye; and this appears to me the more evi- 

 dent, because, when placed on a black ground, which can- 

 not reflect any luminous rays, these irises entirely disappear* 

 The account of this phenomenon, which 1 shall give below, 

 I hope will leave no doubt on this theory of the irises, that 

 accompany small bodies seen at a very short dibtance. 

 These experiments tlien are so far from atibrding proofs of 

 the imperfection of tlie achromatic system of the eye, that 

 X deduce from them arguments i'or its great perfection. 

 Penumbra of ^ '^^ penumbra of small objects seen indistinctly, its exf 

 siiiall objects tent, and the irregqlarity of its colour, depend on the uq- 

 ieenindisanct- , p ,- /> ^i i • ^i ^ • ^ ^i 



lynotcausd equal retraction oi the luminous rays, that arrive at the 



by the refrac- transparent cornea with different degrees of iticiination, and 

 the «ye. a diverging direction. It is evident, that, if the heteroge- 



neous rays be unequally refracted by the humours of the 

 eye, spots of different colours, seen at equal distances^ 

 should exhibit penumbras unequal in extent, and propor- 

 tional to the refractive |,owers of each kind of rays they re- 

 flect. Now this is contradicted by experience. Little cir- 

 cular pasteboards, perfectly equal in size and three milli- 

 meters [1*18 line] in diameter, painted red, yellow, blue, 

 and green, seen on black and white grounds, at equal dis- 

 tances, and at one tune, constantly exhibited penumbraA 

 equal in extent. 

 Experiments The second argument opposed to the theory of the per- 

 wiih the opio- fection of the achromatic system of the eye is derived from 



experiment 



