FUNCTION AND PARTS OF THE MICROSCOPE. 21 



microscope, commonly two eyepieces and two or more 

 objectives of different magnifying powers. The lenses of 

 some makers are designated by arbitrary symbols; as a 

 rule, however, they are marked with numerals w;hich 

 indicate what is known as their equivalent focus, or the 

 focal length of a simple converging lens which would 



Fig. 1 6. — Relation or Distance from Principal Focus to Size 

 AND Position of Image. 



produce an image of the same size as that formed by 

 the lens in question. The larger the number the lower 

 will be the power. Thus a i|-in. eyepiece magnifies 

 less than a i-in. eyepiece, and a J-in. objective less than 

 a ^-in. objective. The commonest high-power objec- 

 tive is the i^j, and the f 5 is the highest objective which 

 can be practically constructed. By making use of ultra- 

 violet rays with a wave-length only half that of ordinary 

 light the Zeiss Optical Company has recently been able 

 to construct instruments of much higher power. The 

 lenses in this case are of fused quartz, and the image 

 produced, being of course invisible, is recorded on the 

 photographic plate. 



