Chap. 9.] the Eye an achromatic Inftrumtnt. -269 

 and thus all meet together at the point #, or very near- 

 ly fo *.' 



The more we inveftigate the works of nature, the 

 greater reafon have we to admire the wifdom of its 

 auth6r, and that wonderful adaptation of our organs, 

 in the minuted particulars, to the general laws which 

 pervade the univerfe. The fubject before us affords 

 a ftriking inftance to corroborate 1 this remark. We 

 have hitherto fuppofed the eye to be a lens capable 

 only of enlarging and contracting, and confequently, 

 from the defcription now given of the rays of light, it 

 muft be incapable of obviating the confulion which 

 muft arife from their different degrees of refrangibi- 

 lity. But here the ufe of that wonderful ftructure of 

 parts, and the different fluids in the eye, is clearly feen. 

 The eye is, in fact, a compound lens. Each fluid has 

 its proper degree of refrangible power. The fhape of 

 the ienfes is altered at will, according to the diftance 

 of the object; and the three fubftances having the pro- 

 per powers of refrangibility, the effects of an achro- 

 matic glafs are without difficulty performed by the 

 eye, whofe mechanical ftructure arid judicious arrange- 

 ment of fubftances it is in vain for the art of man to 

 imitate. 



From what has been ftated, the principal pheno- 

 mena of colours may, without much difficulty, be ex- 

 plained. 



If all the different coloured rays which the prifm 

 affords are re-united by means of a concave mirror, 

 the produce will be white; yet tl)efe fame rays, which, 

 taken together, form white, give, after the point of 

 their re-union, that is, beyond the point where they 

 crofs each other, the fame colours as thofe which de- 



* Encyclop. Brit. vol. xiii. p. 354. 



parted 



