286 PEACTICAL PHOTO-MICEOGEAPHY 



illuminant was arranged in a position to one side of and about 

 fifteen inches away from the preparation, so that a beam of 

 light at about an angle of 45 to the normal was projected on to 

 it. An ordinary bull's-eye condenser was interposed, so that 

 an enlarged image of the illuminant was roughly focussed on 

 to the preparation. 



Exposure, 1J minutes. 



Thomas's lantern-plate. 



Development with pyro-ammonia, Thomas's formula. 



Such low-power opaque objects present no particular 

 difficulty, except that the preparation should be mounted so 

 that all the units lie in as nearly as possible the same plane. 

 In cases where the object is one of considerable depth, it is 

 better to have recourse to a lens of the planar type which has 

 an iris-diaphragm. This allows the aperture to be reduced, 

 with resulting increase of penetration. Alternatively, a Davis 

 diaphragm may be placed at the back of the micro-objective, 

 and this can be closed down as required, care being taken that 

 the quality of the image is not interfered with. In any case, 

 for reasons already stated, the lens should be of the lowest 

 power possible. 



It may be necessary to vary the angle of incidence of the 

 light according to the depth of the surface-markings on each 

 individual unit of the preparation. The nearer the incident- 

 beam approaches to the vertical, the less the contrast that will 

 be obtained. It is, however, always advisable to keep the 

 incident beam at as small an angle to the vertical as practicable ; 

 otherwise heavy shadow effects are caused on one side, which 

 interfere seriously with correct delineation of the object. 



PLATE II, FIG. 2. Transverse Section of Stem of Sar- 



saparilla. X 50. Stained with Bismarck brown. 



(Owing to the colour of this stain, this object maybe regarded 

 as one which presents no colour difficulty in photographing, 

 for the colour of the image itself is non-actinic to any ordinary 

 plate.) 



Objective, 1 inch, by C. Baker. 



No ocular. 



No sub-stage condenser. 



Illuminant, an acetylene bicycle lamp. Ordinary bull's- 

 eye condenser projecting a convergent beam, and so placed 



