CH. VITI-] PHOTO-MICROGRAPHY 233 



nifieation produced by the projection ocular and increase the time ac- 

 cordingly ; thus when the X4 ocular is used, the time should be at 

 least doubled over that when no ocular is employed. The time will be 

 still further increased if a color screen is used (§ 359). 



Zeiss recommends that when the bellows have sufficient length the 

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

 ones. It is also sometimes desirable to limit the size of the field by 

 putting a smaller diaphragm over the eye lens. This also aids in 

 making the field uniformly sharp. 



§ 373. Determination of the Magnification of the Photo- 

 Micrograph. — 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 meas- 

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

 the size of the image by the known size of the object (§ 154). If now 

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

 on a record table like that in section 360, one can get a close approxi- 

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

 bination 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 millimeter ruled with fine lines for the higher powers. 



§ 374. Photo-Micrographs at a Magnification of 100 to 150 

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

 section will answer, but the objectives must be of shorter focus, 8 to 3 

 mm. It is better, however, to use an achromatic condenser instead of 

 the engraving glass or the Steinheil lens. 



§ 375. Lighting for Photo-Micrography with Moderate and 

 High Powers. — ( 100 to 2,500 diameters). No matter how good one's 

 apparatus, successful photo-micrographs cannot be made unless the ob- 

 ject 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 on pp. 41-52. Then for each picture the photographer must 

 take the necessary pains to light the object properly. An achromatic 

 condenser is almost a necessity (§ 80). Whether a color-screen should 



