228 PHOTOGRAPHING WITH THE MICROSCOPE [Ch. VII 



It is recommended that when the bellows have sufficient length 

 the lower projection oculars be used, but with short bellows the 

 higher ones. 



§ 350. Determination of the magnification of the photo-micro- 

 graph. — After a successful negative has been made, it is desirable 

 and important to know the magnification. This is easily determined 

 by removing the object and putting in its place a stage micrometer. 

 If the distance between two or more of the lines of the image on the 

 focusing screen is obtained with dividers and the distance measured 

 on one of the steel rules, the magnification is found by dividing the 

 size of the image by the known size of the object (§ 234). If now the 

 length of the bellows from the tube of the microscope is noted, say 

 on a record table like that in § 358, one can get a close approxi- 

 mation to the power at some other time by using the same optical 

 combination and length of bellows. 



For obtaining the magnification at which negatives are made it is 

 a great advantage to have one micrometer in half millimeters ruled 

 with coarse lines for use with the lower powers, and one in 0.1 and 

 0.01 millimeters ruled with fine lines for the higher powers (fig. 80). 



§ 351. Photo-micrographs at a magnification of 100 to 150 diam- 

 eters. — For this, the simple arrangements given in the preceding 

 section will answer, but the objectives must be of shorter focus, 8 

 to 3 mm. 



§ 352. Lighting for photo-micrography with moderate and high 

 powers. (100 to 2500 diameters). — No matter how good one's 

 apparatus, successful photo-micrographs cannot be made unless the 

 object to be photographed is properly illuminated. The beginner 

 can do nothing better than to go over with the greatest care the 

 directions for centering the condenser, for centering the source of 

 illumination, and the discussion of the proper cone of light and lighting 

 the whole field, as given in § 110-112. Then for each picture the 

 photographer must take the necessary pains to light the object prop- 

 erly. An achromatic condenser is almost a necessity (§ 101). Whether 

 a color screen should be used depends upon judgment and that can 

 be attained only by experience. In the beginning one may try with- 

 out a screen and with different screens and compare results. 



