THE EYE. 305 



property of refracting the ray, the image is carried 

 farther off to a ; and by this means it is ordered 

 that wherever the ray penetrates, it shall be 

 drawn to an accurate focus. 



Some modern philosophers have asserted that 

 the eye is not perfectly achromatic in every adjust- 

 ment. The term implies the property of the in- 

 strument to represent an image divested of the 

 prismatic colours ; those false colours which attend 

 the refraction of the rays of light. If the state- 

 ment be correct, it is nothing against our argu- 

 ment ; nor have those inquirers advanced it with 

 any such view.* We know that in all the ordi- 

 nary exercises of the eye the image is perfect, 

 having neither penumbra nor prismatic colours. 

 This property of the eye results from the different 

 media through which the rays are transmitted, 

 and the gradual transmission which we have just 

 mentioned. Dollond's achromatic glasses, a great 

 improvement upon the telescope, were made on 

 this principle. He composed the object-glass of 

 the telescope of crown-glass and flint-glass, so 

 that while, by the combined effect of their con- 

 vexities, they drew the rays to a focus, the dis- 

 persive power of the one was counteracted by 

 that of the other. 



* Professor Blair (Edrnftwrg-^ Transactions^, in.) expressly derives 

 an argument in favour of design from this statement of his opin- 

 ion, and his objection to Boscovich. 



27* 



