CH. VIII} PHOTO-MICROGRAPHY 235 



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

 and lighting the whole field, as given in § 94, 106. Then for each 

 picture the photographer must take the necessary pains to light the 

 object properly. An achromatic condenser is almost a necessity 

 (§ 91). Whether a color-screen should be used depends upon 

 judgment and that can be attained only by experience. In the be- 

 ginning one may try without a screen, and with different screens 

 and compare results. 



A plan used by many skilled 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. This metal diaphragm 

 must be removed while focusing the diaphragm of the projection 

 ocular, as the diaphragm opening is 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. 



§ 311. Adjusting the Objective for Cover-Glass. — After 

 the object is properly lighted, the objective, if adjustable, must be 

 corrected for the thickness of cover. If one knows the exact thick- 

 ness of the cover and the objective is marked for different thick- 

 nesses, it is easy to get the adjustment approximately correct 

 mechanically, then the final corrections depend on the skill and 

 judgement 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 practi- 

 cally extended to the focusing screen and as the effect of lengthening 

 the tube is the same as thickening the cover-glass; the adjusting 

 collar must be turned to a higher number than the actual thickness 

 of the cover calls for (see § 113). 



§ 312. Photographing Without an Ocular. — Proceed ex- 

 actly as described for the lower power, but if the objective is ad- 

 justable make the proper adjustment for the increased tube-length 



(§ii3-) 



§ 313. Photographing with a Projection Ocular. — Proceed 



