optics. 249 



This appearance is also seen by moon light, 

 though seldom vivid enough to render the colours 

 very distinguishable ; and the artificial rainbow may 

 be produced even by candle-light, on the water 

 which is ejected by a small fountain, or jet d'eau. 

 All these are of the same nature, and dependent on 

 the same causes, viz. the various refrangibility of 

 the rays of light. 



The colours observable on soap-bubbles, and the 

 halos which sometimes surround the moon, are also 

 referable to the same origin. 



This unequal refraction of the rays of light 

 proves a great inconvenience in the construction 

 of single lenses, as the images formed by them 

 have always coloured rays round them, owing to the 

 separation of the coloured rays at the extremities of 

 the lens. This has given rise to a very ingenious 

 construction of a lens, compounded of three others 

 fitted together, and made of glass having different 

 degrees of refrangibility, so that the refractive 

 power of one lens shall correct that of the other, 

 and thus produce an image free from the colours 

 occasioned by common lenses; these compound 

 lenses are called achromatic glasses, and are usually 

 composed of two concave lenses of crown-glass, 

 with a concave lens of flint-glass between them. 

 These were invented by Dollond, and are now 

 used in all the best telescopes. 



Description of the Eye, and the Nature of Vision. 



In order to understand the science of vision, it 

 is necessary, first, to describe the organ of sight. 



The eye is placed in a bony cavity called the 

 orbit, which is lined with fat, to form a soft bed 



