LIGHT. TOf 



aqueous humour. In this there is a membrane called the 

 ttvea, with a hole in the middle, termed the pupil, of a 

 muscular texture, for altering the dimensions of the hole 

 so as to regulate the quantity of light to he admitted. 

 Behind this membrane is a substance of considerable 

 consistency, and formed like a lens, called from its trans- 

 parency the crystalline humour ; this is contained in a 

 fine tunic, called the choroides, and by means of a set of 

 muscles may be moved a little nearer to the bottom of 

 the eye, or further from it, to alter the focal distance, as 

 necessity may require. The remaining part of the eye is fill- 

 ed with a jelly-like substance called the vitreous humour; 

 and at the back is the retina, which is an expansion of the 

 optic nerve, on which every thing that is seen is painted, 

 and conveyed by means of the optic nerve to the brain. 

 The delineation of objects on the retina is prettily de- 

 monstrated by taking a bullock's eye quite fresh, and, 

 having stripped off the skin and the fat from the back 

 part until only the thin membrane behind remains, 

 placing it before any object, and the picture of the ob- 

 ject will appear on the membrane, which is the retina 

 or the membrane may be taken away, and a piece of 

 white paper substituted in its stead ; the object will then 

 appear on the paper, but in either case it will be inverted 

 in the same manner as it would have been had the animal 

 been alive. As the object seen is always painted in an 

 inverted position on the retina, it very probably becomes 

 again reverted as it ascends the optic nerve. It is most 

 wonderful that a prospect of many miles in extent, diver- 

 sified with hill and dale, houses and fields, men and ani- 

 mals, can be painted on so small a space as the retina of 

 the eye. It may be observed, that the magnitude of the 

 object seen, depends on the angle subtended by the ob- 

 ject ; consequently, were it not for the judgment, we 

 should not have any correct idea of the relative size of 

 bodies. As the angles under which bodies are seen in- 

 crease and decrease in a direct ratio, according to the 

 distance, a man fifty yards distant is depicted on the 

 retina twice* as large as a man one hundred yards dis- 



* It would be probably more correct to say four times, as he will 

 appear twice as high, and twice as broad. _ 



