CM. VIII.} PHOTO-MICROGRAPHY. 201 



^ 341. Photo- Micrographs at a Magnification of 100 to 150 Di- 

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

 section will answer, hut 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. 



§ 342. 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. 39 to 46. Then for each picture the photographer 

 must take the necessary pains to light the object properly. An achro- 

 matic condenser is almost a necessity (§ 76). Whether a color-screen 

 should be used depends upon judgment and that can be attained only 

 by experience. In the beginning one may try without a screen, and 

 with different screens and compare results. 



A plan used by many skillful workers is to light the object and the 

 field around it well and then to place a metal diaphragm of the proper 

 size in the camera very close to the plate holder. This will insure a 

 clean, sharp margin to the picture. Of course this metal diaphragm 

 must be removed while focusing the diaphragm of the projection 

 ocular, as the diaphragm opening would be smaller than the image of 

 the ocular diaphragm. 



If the young photo-micrographer will be careful to select for his first 

 trials objects of which really good photo-micrographs have already 

 been made, and then persists with each one until fairly good results are 

 attained, his progress will be far more rapid than as if poor pictures of 

 many different things were made. He should, of course, begin with 

 low magnifications. 



§ 343. Adjusting the Objective for Cover-Glass. — After the ob- 

 ject is properly lighted, the objective, if adjustable, must be corrected 

 for the thickness of cover. If one knows the exact thickness of 

 the cover and the objective is marked for different thickness, it 

 is easy to get the adjustment approximately correct mechanically, 

 then the fine or final correction depends on the skill and judg- 

 ment of the worker. It is to be noted too that if the objective is 

 to be used without a projection ocular the tube length is practically ex- 

 tended to the focusing screen and as the effect of lengthening the tube 

 is the same as thickening the cover-glass, the adjusting collar must 



