OPTICAL INSTRUMENTS. 



361 



which the object 13 seen — since the apparent magnitude 

 of every body increases or diminishes with the size of this 

 angle. 



Microscopes are of two kinds — simple and compound. 

 What are the ^^ *^^ simplc microscopc, the object under 

 two ▼arieties examiuatiou is viewed directly, either by a 



•f microscopes? . •' - •' 



Simple or compound convergmg lens. 

 In the compound microscope, an optical image of tha 

 object, produced upon an enlarged scale, is thus viewed. 



The simple microscope is generally a simple convex lens, in the locus of 



which the object to be examined 

 is placed. Little spheres of glass, 

 formed by melting glass threads 

 in the flame of a candle, form 

 very powerful microscopes. 



Fig. 293 represents the mag- 

 nifying principle of the micro- 

 scope. An eye at E would seo 

 the arrow A B, under the visual 

 angle A E B ; but when tho 

 lens, F F' is interposed, it is 

 seen under the visual angle at 

 A' E B', and hence it appears 

 much enlarged, as ahown in the image A' B'. 



FlQ. 293. 



Fig. 294 represents the most im- 

 proved form of mounting a simple 

 microscope. A horizontal support, 

 capable of being elevated or depressed 

 by means of a screw and ratch-woik, 

 D, sustains a double-convex lens, A. 

 The object to be viewed is placed 

 upon a piece of glass, C, upon a stand- 

 ard, B, immediately below the lens. 

 As it is desirable that the object to 

 be magnified should be strongly 

 illuminated, a concave mirror of glass, 

 M, is placed at the base of the instru- 

 ment, inclined at such an angle as to 

 reflect the rays of light which fall 

 upon it directly upon the object. 



What is the 720. TheCom- 



Zt'ZTouM pound Micro- 



Microscope ? g^jQpg^ i^ jj.g Jj^^st 



FiS. 294. 



