186 METHODS OF MICROSCOPICAL RESEARCH. 



be, or simply wedged) into the arm that usually 

 carries the tube. By this means the field is only 

 limited by the aperture of the objective, and there 

 is no possibility of flare from the tubes. 



A mechanical stage, with concentric rotation, 

 will greatly facilitate the adjustment of the picture 

 in the centre of the plate. With objectives of 

 higher power than -| inch, these mechanical motions 

 become indispensable. Spring clips are required 

 to keep the object in a vertical position. 



The camera may be of a size to take plates from J 

 to whole plate size according to the worker's choice, 

 but at first, at all events, if not permanently, the 

 smaller sizes are best. The quarter plate size is 

 most generally used, and plates of that size are 

 always obtainable and are cheap, but we would 

 recommend 5 by 5 inches as a more useful size, con- 

 forming more nearly to the circular form of the 

 field, and to many objects, diatoms, echinus spines, 

 sections of stems, &c., which involve a great waste 

 of surface when taken on oblong plates. By means 

 of a simple adapter a camera of this size will take 

 ^ plates when desired, and these can be placed with 

 their longer edges either horizontal or vertical. 



The camera should be made with a bellows body 

 capable of closing up to 6 or 7 inches and extending 

 to about a yard, it will then be available for use, 

 either with or without the eye-piece. To the front 

 should be fitted the base of a smaller bellows of 

 conical form, whose small end terminates in a wood, 

 metal or cardboard ring, lined with black velvet 

 and fitting smoothly and light-tight on to the 

 outside of the tube of the microscope. The dark 

 slide which carries the plate should be single, and 



