Vision and Optical Glasses. 191 



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 show how much the glass mag- 

 nifies the diameter of the object. The most 

 powerful single microscopes are very small globules 

 of glass, which any person may make for himself 

 by melting the ends of fine glass threads in the 

 flame of a candle. 



The double or compound microscope consists 

 of an object-glass cd, (fig. 72,) and an eye-glass ef'. 

 The small object ab is placed at a little greater 

 distance from the glass cd than its principal focus, 

 so that the pencils of rays flowing from the dif- 

 ferent 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 gli 

 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 and^ till they come to the eye at A:, 

 where they will begin to converge by the re- 

 fractive power of the humours ; and after having 

 crossed" each other in the pupil, and passed 

 through the crystalline and vitreous humours, 

 they will be collected into points on the retina, 

 and there form the large inverted image AB. 



