OF OPTICS. 245 



under any sensible angle. The method of viewing such LECT. 

 minute objects is by a microscope, of which there are three ^^-v~x^ 

 sorts viz. the single, the double, and the solar. 



The single microscope is only a small convex glass, as The * 

 , i . ., ,. , J , , . .. microcctpe. 



c a, having the object a b placed m its 

 focus, and the eye at the same distance 

 ou the other side ; so that the rays of each 

 pencil, flowing from every point of the ob- 

 ject on the side next the glass, may go 

 on parallel in the space between the eye 

 and the glass ; and then by entering the eye at C, they 

 will be converged to as many different points on the re- 

 tina, and form a large inverted picture A B upon it, 

 as in the figure. 



To find how much this glass magnifies, divide the 

 least distance (which is about six inches) at which an 

 object can be seen distinctly with the bare eye, by the 

 focal distance of the glass ; and the quotient will shew 

 how much the glass magnifies the diameter of the object. 



The double or compound microscope, consists of an ob- 



* 11 i mcroscupr 



ject-glass c a, and an eye-glass ej. Ine small object 

 a b is placed at a little 

 greater distance from the 

 glass c d than its principal 

 focus, so that the pencils 

 of rays flowing from the 



different points of the object, and passing through 

 the glass, may be made to converge and unite in 

 as many points between g and h, where the image 

 of the object will be formed : which image is 

 viewed by the eye through the eye-glass ef. For the 

 eye-glass being so placed, that the image g h may be in 

 its focus, and the eye much about the same distance on 

 the other side, the rays of each pencil will be parallel, 

 after going out of the eye-glass, as at e audj', till they 

 come to the eye at k where they will begin to con- 

 verge by the refractive power of the humours; and af- 



