236 PHOTO-MICROGRAPHY [CH. VIII 



as described in § 307 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. 



§ 314. Photo-Micrographs at a Magnification of 500 to 

 2000 Diameters. — For this the homogeneous immersion objective 

 is employed, and as it requires a long bellows to get the higher 

 magnification with the objective alone, it is best to use the pro- 

 jection oculars. 



For this work the directions given in § 307 B must be followed 

 with great exactness. The edge of the petroleum lamp flame is suf- 

 ficient to fill the field in most cases. With many objects the time 

 required with good lamp light is not excessive ; viz. , 40 seconds 

 to 3 minutes. The reason of this is that while the illumination 

 diminishes directly as the square of the magnification, it increases 

 with the increase in the numerical aperture, so that the illuminating 

 power of the homogenous immersion is great in spite of the great 

 magnification (§ 40). 



For work with high powers a stronger light than the petroleum 

 lamp is employed by those doing considerable photo-micrography. 

 Good work may be done, however, with the petroleum lamp. 



It may be well to recall the statement made in the beginning, 

 that the specimen to be photographed must be of special excellence 

 for all powers. No one will doubt the truth of the statement who 

 undertakes to make photo-micrographs at a magnification of 500 to 

 2000 diameters. 



If one has a complete outfit with electric arc light the time re- 

 quired for photographing objects is much reduced, i. e. ranging from 

 1 to 20 seconds even with the color screen. As the light is so in- 

 tense with the arc light it is necessary to soften it greatly for focus- 

 ing. Several thicknesses of ground glass placed between the lamp 

 and the microscope will answer. These are removed before taking 

 the negative. It is well also to have a water bath on the optical 

 bench to absorb the heat rays. This should be in position constant- 

 ly (see Fig. 133, 160). 



§315. Use of Oculars in Photo- Micrography. — There is 

 much diversity of opinion whether or not the ordinary oculars used 



