28 THE EYE 



large ; at a distance of two inches and a half, four times ; 

 at one tenth, a hundred times as large, and so on ; in 

 a word, the enlargement depends alone upon the degree of 

 proximity to the eye into which the object is brought. 

 In former days, these simple microscopes were used very 

 extensively and almost exclusively for the purposes of 

 science, because the compound microscopes were then so 

 bad as to be far inferior to the simple instrument The 

 celebrated Leuwenhoek made all his wonderful microscopic 

 observations with simple spheres of glass, which he made 

 for himself, by melting fine threads of glass in a lamp. In 

 the present day, the simple microscope is generally used 

 where very small magnifying power is required, the com- 

 pound instrument where any considerable enlargement is 

 desirable. While the latter fatigues the eye comparatively 

 little, observation with the simple microscope, especially with 

 high magnifying power, is so great a strain upon it, that 

 disease of the eye is but too frequently the result. 



The principle of the compound microscope is also very 

 easily explained. It depends on a combination of the 

 camera obscura with the simple microscope. The common 

 camera obscura consists, essentially, of some glasses ground 

 into the shape of a lens ; the rays of light proceeding from 

 an object pass through these glasses and produce behind 

 them a picture of the object, which is usually received, in 

 the common optical toy, upon a plate of ground glass or a 

 surface of white paper. The farther the object is removed 

 from the glasses, the smaller appears the picture. Bringing 

 the object nearer, the picture grows till picture and object 

 are of equal size. But if the object is now placed closer to 

 the glasses, the picture becomes larger than the object. 

 We never use the camera obscura in this condition, but 

 we do the magic lantern, which in its essential construction 



