64 'On the UNEQUAL 



M. Boscovich formed an hypothecs concerning a perfect 

 correction of colour, by a combination of mediums, which ap- 

 pears to have greatly mined him. As a combination of two 

 mediums is necefTary to unite two of the unequally refrangible 

 rays, he imagines three mediums necefTary to unite three, four 

 to unite four, and, in fhort, that to effect a perfect union of the 

 rays of the fpectrum, as many mediums are required as there 

 are unequally refrangible rays compofing it, that is to fay, an 

 indefinite number. He fuppofes, however, than an union of 

 three of the rays only, by means of three mediums, would 

 greatly improve telefcopes. This author feems to have founded 

 his hypothefis on the fame kind of loofe analogical reafoning, 

 which had before led the celebrated Euler into a fimilar mi- 

 ftake. 



The eye is compofed of three humours and feveral coats ; 

 and M. Boscovich takes it for granted, that a more perfect 

 union of the rays than what takes place in a combination of 

 crown and flint glafs, is effected by their means. But this is 

 a fuppofition very remote from the truth indeed. So far is this 

 fecondary colour from being corrected in the human eye, that 

 in the conftruction of this admirable organ, it hath been 

 deemed unneceffary to introduce any contrivance for the cor- 

 rection of the Newtonian aberration. Natura nihil agit fmjlra. 

 The perfection of the Contriver equally appears from a mani- 

 feftation of his power, and of his ceconomical exertion of that 

 power. On account of the fhortnefs of the focal diftance of 

 the humours of the eye, in proportion to the aperture of the 

 pupil, the aberration from the fpherical figure would be enor- 

 mous ; and we find it obviated by the very elaborate artifice of 

 rendering the chryftalline humour more denfe towards the cen- 

 tre. The aberration from difference of refrangibility might 

 have been removed, by imparting a proper degree of difperfive 

 power to the vitreous humour. But this, being unneceffary for 

 the common purpofes of life, is withheld. 



Dr 



