GENERAL RULES FOR THE CONSTRUCTION OF STANDS. 119 



in two ways : (a) with regard to the aperture of the incident 

 cone of light, and (b) with regard to its inclination. A perfect 

 illuminating apparatus must therefore include means for con- 

 trolling both these conditions. 



In most cases a plane or a concave mirror of ordinary size 

 satisfies these requirements if it can be brought sufficiently near 

 to the object, and admits, moreover, of lateral movement. If the 

 light thus obtained is still too weak, the convergence of the rays 

 may be increased by condensing lenses. The limitation of the 

 incident cones of light is effected in both cases by diaphragms 

 attached below the object-stage, or sunk in it. 



Large Microscopes are generally furnished with plane and con- 

 cave mirrors in one and the same mounting, which can be moved 

 in all directions, or at least in the vertical plane. The smaller 

 instruments usually have a concave mirror only, which revolves 

 on its own centre, and is at the same time movable in the direction 

 of the axis ; this simplification in the mounting of the mirror 

 necessitates the use of special means for oblique illumination. 

 A still greater restriction of the movements of the mirror as we 

 find, for instance, in the small drum stands (stands with cylinder- 

 base) is, generally speaking, inconvenient. 



With reference to the apparatus for the illumination of opaque 

 objects, we must refer our readers to the statements made above. 



-Diaphragms. It is obvious that a small diaphragm-aperture, 

 say at the distance of a millimetre from the plane of adjustment, 

 limits the incident cones of light like an aperture of double the 

 diameter at double the distance. Theoretically it is therefore quite 

 immaterial in what manner, or even in what plane between mirror 

 and object, the diaphragms are applied, provided only the size of 

 the aperture is in the proper ratio to its distance from the object. 

 In practice, however, the diaphragm-apparatus should be capable 

 of being easily and conveniently used ; it should permit the most 

 different gradations of the illumination without displacing the 

 object ; and, moreover, it should completely exclude all extraneous 

 light reflected by the work-table or stand. For this purpose 

 revolving discs with a series of apertures were originally applied 

 below the stage, which were at the same time movable in a vertical 

 direction. The apertures were so arranged that when the disc 

 was revolved they coincided successively with the optic axis. An 

 arrangement of this kind, accurately constructed, answers most 



