226 PHOTO-MICROGRAPHY {CH. VIII 



(\ 290) almost any preparation which shows the elements clearly when look- 

 ing into the microscope can be satisfactorily photographed. Good photo- 

 graphs cannot, however, be obtained from poor preparations. 



\ 304. Light. — The strongest light is sunlight. That has the defect of 

 not always being available, and of differing greatly in intensity from hour to 

 hour, day to day and season to season. The sun does not shine in the evening 

 when many workers find the only opportunity for work. Following the sun- 

 light the electric light is the most intense of the available lights. Then come 

 magnesium, acetylene, the lime light, the gas-glow or Wellsbach light and 

 petroleum light. The last is excellent for the majority of low and moderate 

 power work. And even for 2 mm. homogeneous immersion objectives, the 

 time of exposure is not excessive for many specimens (40 seconds to 3 

 minutes). This light is cheap and easily obtained. It has the advantage of 

 being somewhat yellow, and therefore in many cases makes the use of a color 

 screen unnecessary if one uses isochromatic plates. 



A lamp with fiat wick about 40 mm. (ij^ in.) wide has been found most 

 generally serviceable. For large objects and low powers the flame may be 

 made large and the face turned toward the mirror. This will light a large 

 field. For high powers the edge toward the mirror gives an intense light. 

 The ordinary glass chimney answers well, especially where a metal screen is 

 used as shown in Fig. 171. 



EXPERIMENTS IN PHOTO-MICROGRAPHY 



§ 305. The following experiments are introduced to show 

 practically just how one would proceed to make photo- micrographs 

 with various powers, and be reasonably certain of fair success. If 

 one consults prints or the published figures made directly from 

 photo-micrographs it will be seen that, excepting diatoms and 

 bacteria, the magnification ranges mostly between 10 and 150 

 diameters. 



§ 306. Focusing in Photo-Micrography. — For rough focus- 

 ing and as a guide for the proper arrangement of the object one 

 uses a ground-glass screen as in gross photography. With the 

 ground-glass screen one can judge of the brilliancy and evenness of 

 the illumination more accurately than in any other way. For final 

 and exact focusing two principal methods are employed : 



(A). A focusing glass is used either with a clear screen or in a 

 board screen as described above (§ 282). The latter method is like 

 focusing with the compound microscope and a positive ocular. If 

 the focusing glass is set properly the focus should be easily aud 

 accurately determined. 



