VISION. 475 



eye, by the nice adjustment of the powers of the 

 different refracting media, which the rays of 

 Hght have to traverse before they arrive at the 

 retina, producing what is called an achromatic 

 combination ; * and it is found that the eye, 

 though not an absolutely achromatic instrument, 

 as was asserted by Euler,t is yet sufficiently so 

 for all the ordinary practical purposes of life. 



The object, then, of the whole apparatus ap- 

 pended to the optic nerve, is to form inverted 

 images of external objects on the retina, which, 

 as we have seen, is the expanded extremity of 

 that nerve. That this effect is actually pro- 

 duced, may be easily shown by direct obser- 

 vation ; for if the sclerotic and choroid coats be 

 carefully dissected off from the posterior part of 

 the eye of an ox, or any other large quadruped, 

 leaving only the retina, and the eye so prepared 

 be placed in a hole in a window-shutter, in a 

 darkened room, with the cornea on the outside, 

 all the illuminated objects of the external scene 

 will be beautifully depicted, in an inverted posi- 

 tion, on the retina. 



Few spectacles are more calculated to raise 

 our admiration than this delicate picture, which 



* For the exposition of the principles on which these achro- 

 matic combinations of lenses correct this source of aberration, I 

 must refer to works which treat professedly on Optics, 



t For the rectification of this error we are indebted to Dr. 

 Young. 



