Chap. 8.] Camera Otycura. 259 



ing piece in which the glafs is fet or fixed. This il- 

 lumination is ftill more increafed by the reflexion of 

 light from a concave mirror S S, placed at the other 

 end of the box, that caufes the light to fail upon the 

 lens N N, as reprefented in the figure. Laftly, a lens 

 L L, fixed in a Hiding tube, is brought to the requifite 

 diftance from the object O B, and a large erect image 

 I M is formed upon the oppofite wall. 



The camera obfcura has the fame relation to the 

 telefcope, as the folar microfcope has to the common 

 double microfcope, and is thus conftructed : 



Let C D (fig. 48.) reprefent a darkened chamber 

 perforated at L, where a convex lens is fixed, the cur- 

 vity of which is fuch, that the focus of parallel rays 

 falls upon the oppofite wall. Then if A B is an ob- 

 ject at fuch a diftance, that the rays which proceed 

 from any given point of its furface to the lens L may 

 be efteemed parallel, an inverted picture will be formed 

 on the oppofite wall ; for the pencil which proceeds 

 from A will converge to a, and the pencil which pro- 

 ceeds from B will converge to b t and the interme- 

 diate points of the object will be depicted between 

 * and b *. 



' * Nicholfon's Natural Philof. vol. i. p 347- 



