234 PHOTO-MICROGRAPHY \_CH. VIII 



be used depends upon judgment and that can be attained only by ex- 

 perience. In the beginning 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 dia- 

 phragm. 



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. 



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

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

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

 of the cover and the objective is marked for different thicknesses, it is 

 easy to get the adjustment approximately correct mechanically, then 

 the final corrections depend on the skill and judgment of the worker. 

 It is to be noted too that if the objective is to be used without a projec- 

 tion ocular the tube-length is practically 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 § 103). 



§ 377. Photographing Without an Ocular. — Proceed exactly 

 as described for the lower power, but if the objective is adjustable make 

 the proper adjustment for the increased tube-length (§ 103). 



§ 378. Photographing with a Projection Ocular. — Proceed as 

 described in § 372 B, only in this case the objective is not to be adjusted 

 for the extra length of bellows. If it is corrected for the ordinary 

 ocular, the projection ocular then projects this correct image upon the 

 focusing screen. 



§ 379. Photo-Micrographs at a Magnification of 500 to 2000 

 Diameters. — For this the homogeneous immersion objective is em- 

 ployed, and as it requires a long bellows to get the higher magnifica- 

 tion with the objective alone, it is best to use the projection oculars. 



