238 FERGUSON'S LECTURES. 



LECT. cannot be so soon collected into the corresponding 

 \^v-x." points behind it. Consequently, if the distance of the 

 object A B C be equal to the distance e B of the focus 

 of the glass, the rays of each 

 pencil will be so refracted 

 by passing through the 

 glass, that they will go out 

 of it parallel to each other ; 

 as d I, e H, f h, from the 

 point C; d G, e K,J D, from the point B ; and a? K, e E, 

 f L, from the point A ; and therefore, there will be no 

 picture formed behind the glass. 



If the focal distance of the glass, and the distance of 

 the object from the glass, be known, the distance of the 

 picture from the glass may be found by this rule, viz. 

 multiply the distance of the focus by the distance of the 

 object, and divide the product by their difference ; the 

 quotient will be the distance of the picture. 



The picture will be as much bigger or less than the 

 object, as its distance from the glass is greater or less 

 than the distance of the object. For, as B e (in engrav- 

 ing, page 237.) is to e b, so is A C to c a. So that if 

 A B C be the object, c b a will be the picture ; or, if 

 c b a be the object, ABC will be the picture. 

 The man- Having described how the rays of light, flowing from 

 nerofri- o kj ec t g an( j passing through convex glasses, are collect- 

 ed into points, and form the images of the objects ; it 

 will be easy to understand how the rays are affected by 

 passing through the humours of the eye, and are thereby 

 collected into innumerable points on the bottom of the 

 eye, and thereon form the images of the objects which 

 they flow from. For, the different humours of the eye, 

 and particularly the chrystalline humour, are to be con- 

 sidered as a convex glass ; and the rays in passing 

 through them to be affected in the same manner as in 

 passing through a convex glass. 



