232 



PHOTO-MICROGRAPHY [CH. VII 1 



is also made in the same way, although one must have regard to 

 the greater magnification produced by the projection ocular and in- 

 crease the time accordingly ; thus when the X4 ocular is used, the 

 time should be at least doubled over that when no ocular is em- 

 ployed. The time will be still further increased if a color screen is 

 used (§ 294). 



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. 



§ 308. 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 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 (§ 170). If now the length of the bellows from the tube of 

 the microscope is noted, say on a record table like that in section 

 316, one can get a close approximation 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 millimeter ruled with fine lines for the higher powers. 



§ 3°9- 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 con- 

 denser instead of the engraving glass or the Steinheil lens. 



§ 310. 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 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 



