132 PHOTO-MICROGRAPHY 



with low-powers, equally applies to when it maybe employed with high ones. Owing 

 hoN\ever, to no magnification comparatively being obtained by camera length, so 

 subsequent enlargement mmt be resorted to. Dr. Van Heurck does this regularly 

 and considers it we are led to believe the best method. It is certainly easier than 

 photographing directly to say 3000 diameters, but we are doubtful as to the superiority 

 of the result in all cases. 



The use of oblique light is hardly ever reqmred in medium-power work unless it is 

 when employing a sixth. There is little to be described about it save that in applying 

 it to the condenser as shown in another place, p. 94, considerable care has to be exer- 

 cised that diffraction effects produced optically in the specimen may not be mistaken 

 for realities of structure. The auxiliary condenser must be shifted aVjout laterally 

 to produce the best effect, and some form of elliptically or otherwise cut diaphragm 

 (s3e Fig. 48, p. 95, and Fig. 53, p. 100) placed underneath the substage condenser. 



Dark Ground Illumination. — In a previous section (p. 112) it is explained how this 

 is obtained, it only remains to give a few directions as to taking the photograph with 

 its use. Considerable care must be taken to keep the dark ground as clear and free 

 from deposit as possible during development. Adding a little extra bromide from 

 time to time is useful, although it slows the process ; and great additional care 

 should be exercised, especially when using isochrom.atic plates, that the dark room 

 lamp is kept well down and that the dish should be continuously covered, both of 

 which are to prevent any trace of fog. 



Photogf^aphing Opjaque Objects. — The arrangements for this purpose, when requiring 

 a magnification of only a few diameters, has been spoken of (p. 113); but in what follows 

 reference is made to amplifications which require the actual use of the microscope. 



The position of the illuminants is much the same as previously mentioned, but it 

 will be found far more difficult to prevent the shadow of the objective — say when 

 using a half inch — being cast on the object itself In other words equal illumination 

 is most difficult to obtain. Several methods have, however, been adopted. One is 

 the use of a Lieberkuhn, which may be said to consist of a hollow mirror, through 

 whose axis the object glass just protrudes. The light passing through the slip 

 around the specimen in the ordinary fashion — by which is meant that part which 

 is not intercepted by the opaque object or its background — is cast back by the 

 parabolic reflector on to the object to be photographed. 



This arrangement will sometimes give surprisingly fine results, but as often, too, 

 those which are most disappointing ; we cannot recommend it except under exception- 

 ally favourable circumstances, which means that the object and background are just 



