the other. Some find it a great assistance to direct the un- 

 engaged eye upon a dark object, such as a blackened card, 

 which they fasten to the tube of the instrument near its top. 



As it is desirable that the lamp should only illuminate the 

 reflector, a great many ingenious contrivances have been made 

 to cut off the superfluous light. For this purpose some micros- 

 copists interpose a piece of thin board, or a thick card, having 

 a circular opening between the lamp and the reflector. 



Stage diaphragms. When the pencil of light has been 

 reflected from the mirror upon the opening in the stage, it is 

 plain that a larger or smaller amount of light will pass, accord- 

 ing to the size of the opening. The appliances that regulate 

 this matter are called stage diaphragms sometimes they are 

 simply cylindrical tubes with capped upper extremities, each 

 tube being provided with caps of varying aperture. The tubes 

 are pushed into the stage from beneath. When polished they 

 undoubtedly aid in converging the light upon the aperture. 

 Other diaphragms are simply round holes in a circular revolving 

 plate which is set into the stage. 



The diameters of the apertures vary from that of a pin's 

 point to about three-fourths of an inch or even more. 



The revolving diaphragms have now come into general use, 

 because they work simply and efficiently. Mr. Wale has de- 

 vised one that is extremely ingenious. It has the advantage 

 of a cylindrical diaphragm, in so far as it converges the pencil 

 of light upon the diaphragmatic opening, while the size of the 

 opening is regulated by the action of a single thumb-screw.' 

 It acts as the iris does in enlarging or diminishing the pupil, 

 from which its name, the iris diaphragm. 



The mirrors. Of these there should be two, one plane when 

 a diffuse light is needed ; the other concave for a concentrated 

 beam. The latter is frequently used, the former seldom. 



Direct and oblique light. Thus far the descriptions have 

 applied to direct light, and it is the only kind much used in 

 histological work. In testing a lens, however, as with a diatom, 

 it is often necessary to use oblique light in order to resolve a 

 line or series of lines. In such cases the aperture in the stage 

 should be made as large as possible, and the mirror, concave or 

 plane, is to be carried well up under the stage, to the left or 



1 See Appendix A. 



