PRACTICAL PHOTO-MICROGRAPHY. 171 



only because of the magnification required. We have suc- 

 ceeded well in such work with a cheap "student's" oil 

 immersion one-twelfth by Swift, and also with a similar one- 

 twentieth by Keichert, using no ocular in either case ; it is 

 within our knowledge that glasses by Powell and Lealand 

 (specially a twenty-fifth), Beck, and Seibert have been success- 

 fully used in this manner and for the same purpose. The 

 demonstration by photography of flagella, with which some 

 micro-organisms are endowed, depends upon most accurate 

 correction of objective coupled with suitable mounting and 

 staining. Though many of these organisms are flagellated, it 

 is a most difficult matter to keep the flagella visible in mount- 

 ing, and it is little less difficult to photograph them when 

 visible. 



Test diatoms present the usual microscopic difficulties with, 

 generally, a photographic one " thrown in," viz., the difficulty 

 of getting contrast. In these cases angle must not be dis- 

 pensed with, for the great point is not only to get resolution, 

 but to get very strong and distinct resolution. The photo- 

 graphic difficulties must be overcome by photographic skill 

 and not by sacrifice of optical excellence. The diatoms should 

 be absolutely in contact with the cover glass, to determine 

 which Beck's " Vertical Illuminator " is invaluable, and the 

 medium in which the objects are mounted should be of suit- 

 able refractivity. 



It seems to be the fate of every photo-micrographer of any 

 ambition, sooner or later, to attempt to photograph "amphi- 

 pleura pellucida in lines." The first thing is to get a frustule 

 as coarsely marked as possible, but withal clean and flat, 

 mounted in a medium of high refraction index. The " striae " 

 seem usually to run about 95,000 to the inch. The lines can- 

 not with any angular aperture open to us be clearly shown by 

 axial illumination, and oblique illumination is attained by put- 

 ting into the condenser a disc with a slot of one or other of 

 the following shapes : B being preferable in most cases. This 

 slot is variable in breadth and length, and must be chosen to 

 give the best result by experiment. A strong light should be 

 used, as lime light, for under the best conditions of illumination 



