ELEMENTARY PHOTO-MICROGRAPHY. 



73 



metre, or 1-25400111., but the enlargement itself is 

 usually expressed in diameters, that is, by the 

 number of times the picture is longer than the 

 original object, linear measure only being taken 

 and not superficial area. For example, if an object 

 ^jin. long were enlarged to iin., or ten times, it 

 would be said to be magnified ten diameters, 

 usually written x 10. 



TABLE OF APERTURES AND MAGNIFYING POWERS 

 OF OBJECTIVES WITH EYEPIECES. 



OIL-IMMERSION OBJECTIVE. 



POLARISCOPE. 



The polariscope is indispensable for the study 

 of rocks and crystals. This is an instrument for 

 polarising and analysing the light, and generally 

 consists of Iceland spar prisms mounted for easy 



