ELEMENTARY PHOTO-MICROGRAPHY. 59 



This list is taken from the catalogues of various 

 makers, and is not based on any scientific system 

 of uniformity. No reliable nomenclature of eye- 

 pieces and objectives has yet been adopted for the 

 compilation of consistent tables. 



See definition of " Power " in glossary for fuller 

 information. 



The unit for microscopical measurement is the 

 micron (written A*), the thousandth part of a milli- 

 metre, or i-2540oin., 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 

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

 would be said to be magnified ten diameters, 

 usually written x 10. 



Crystals are too transparent for ordinary photo- 

 graphy, but can be brought out by the aid of the 

 polariscope, which shows their form in many 

 colours. It will be found that sober tints yield 

 better negatives than blue and yellow ; the mica 

 or selenite, therefore, can be selected accordingly. 



A body tube dead black inside is essential, and 

 the most suitable powers are 2in. and iin. objec- 

 tives. The eyepiece is dispensed with to com- 

 pensate for the loss of light through the prism. 

 Orthochromatic plates, preferably backed, are used 

 for this class of work, and in rotating the prism a 





