PRACTICAL PHOTO-MICROGRAPHY. 81 



practically colorless. Low power, narrow angle condenser^ 

 magnification about 30 diams. without ocular, 120 with pro- 

 jection ocular. Subject a flat diatom as Arachnoidiscus 

 Ehrenbergii) or an Echinus Spine. Objective two-thirds inch 

 or one inch ; Achromatic condenser, front hemisphere removed. 

 Procedure : A. To centre the condenser (this step must 

 be taken in every case when a substage condenser is to be used.) 

 Placing the pinhole cap on, or a pinhole diaphragm in, the 

 condenser, examine with a low power objective and eye piece, 

 working the substage centering screws, till the disc of light, 

 (which may proceed from any radiant), occupies precisely the 

 centre of the field. Remove pinhole stop or cap. 



B. To centre the light. (This also must ~be performed in 

 every case.) The condenser being centred as under " A," rack 

 out and in the substage and the microscope tube with objective 

 and low power eye-piece, until an image of the light is seen 

 sharp in some part of the field. (The light may require to be 

 moved in order to bring it upon the field.) With an oil lamp 

 the wick should present its edge to the condenser. Move the 

 light from side to side and up and down till its image falls 

 directly in the centre of the field. 



C. To focus objective and condenser on the object. Place 

 the object on the stage so that it occupies the central position, 

 focus the objective on the object and then rack the substage 

 till the image of the light is sharply focused across the object. 

 This is a suitable arrangement so far for obtaining the best 

 possible image microscopically of a certain area and plane of 

 the object. The nearer the light is to the substage, and the 

 longer the focus of the condenser, the larger will be the sharp 

 image of the radiant across the object. 



D. To spread the light evenly over the field either a bull's- 

 eye or a diffusing medium as the ground-glass referred to 

 above must be placed between the light and the condenser. 

 If the " ground-glass and bull's-eye " arrangement be used, the 

 result is simply to transfer the radiant surface from the lamp to 

 the ground-glass ; if the bull's-eye alone be used the result is to 

 fill the back of the condenser with parallel rays ; and in order 

 to get the most evenly lighted field the condenser will probably 



