4:8 PRACTICAL PHOTO-MICROGRAPHY. 



necessities must be attended to ; 1st. The light, the condensing 

 system, the object glass and the centre of the sensitive plate 

 must all be axially centred to each other ; and 2nd. The object 

 and the sensitive plate must be parallel to each other and per- 

 fectly perpendicular to the optical axis of the system. The 

 slightest divergence from either of these relations will entail 

 failure. 



Where the intending photo-micrographer possesses the com- 

 ponent parts of the system, as microscope, lamp, and camera, 

 and requires only to fix these in suitable position on some base 

 to be used permanently, the matter lies chiefly with himself, 

 and the ease of his operations will depend chiefly on the instru- 

 ments he happens to have. We shall figure and describe two 

 arrangements at least ; one a sample of an apparatus sold ready 

 made, the other an apparatus built up of miscellaneous 

 materials by the writer, and these are given merely as sug- 

 gestions and as examples of what has been found to work well 

 in the writer's hands. 



In Britain of late years, several opticians have stocked 

 apparatus of which figure 15 is a type. It presents all the 

 useful features of its class, though other instruments are to be 

 found differing in detail, some details being superior, others in- 

 ferior, to those seen in the cut. 



This apparatus was, the writer believes, designed in the form 

 shown by Professor E. M. Crookshank, and used by him in 

 his photo-micrography of Bacteria. The writer has used an 

 apparatus in all essential points similar to that figured, and 

 had great satisfaction in its use. The stand is so made that 

 when it is necessary to have the stage of the microscope 

 horizontal, as shown in Fig. 15, as when for photography of 

 liquid matter, the base board can be let down to the vertical 

 position ; as every part of the optical and photographic and 

 illuminating systems is clamped to the base board, this position 

 is easily attained ; and, except where a lamp burning same 

 liquid is used, one position is as manageable as the other. If 

 an oil lamp furnishes the light a mirror must come into requis- 

 tion. A point on which the writer lays great stress is : the 

 microscope, condensing system and radiant are all fixed to one 



