CH. VIII} PHOTO-MICROGRAPHY 235 



For this work the directions given in § 372 B must be followed with 

 great exactness. The edge of the petroleum lamp flame is sufficient 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 numerical 

 aperture, so that the illuminating power of the homogeneous immersion 

 is great in spite of the great magnification (§ 34). 



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 especial 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 (Fig. 192) the 

 time required for photographing objects is much reduced, i. e. ranging 

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

 tense with the arc light it is necessary to soften it greatly for focusing. 

 Several thicknesses of ground glass placed between the lamp and the 

 microscope will answer. These are removed before taking the nega- 

 tive. It is well also to have a water bath on the optical bench to ab- 

 sorb the heat rays. This should be in position constantly (see Ch. IX). 



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

 diversity of opinion whether or not the ordinary oculars used for ob- 

 servation should be used in photographing. Excellent results have 

 been obtained with them and also without them. 



For great magnification Zeiss recommends the use of the compen- 

 sation oculars with the apochromatics. 



The Zeiss projection oculars may be used with achromatic objectives 

 of large aperture as well as with the apochromatics. 



PHOTOGRAPHING OPAQUE OBJECTS AND METALLIC SURFACES 

 WITH A MICROSCOPE 



All of the objects cansidered in the first part of this chapter are opaque and 

 some of them were to be photographed somewhat larger than natural size. To 

 meet the needs of modern work, especially with metals and alloys one must be 

 able to examine and photograph prepared surfaces at magnifications ranging from 

 five or ten to five hundred or more diameters. 



\ 381. Microscope for Opaque Objects. — If one does not need to magnify 

 more than about 100 diameters, any good microscope will answer. For the higher 



