THE MICROSCOPE AND ITS ACCESSORIES. 7 



which can be fitted diaphragms of different sizes, or a blackened 

 circular brass plate with holes of different sizes from a pinhole to 

 half an inch rotates under the stage, so that the desired size of hole 

 or diaphragm can be brought under the central aperture in the 

 stage. The aperture in the stage must not be too small ; it should 

 be sufficiently large to enable a section of the spinal cord to be 

 seen as a whole. A small aperture is used with high powers and a 

 large aperture with low powers. In the more expensive, and in 

 some oi the cheaper microscopes also, the stand is provided 

 with a joint, so that the microscope can be inclined as shown in 

 fig. 10. 



5. Illumination of the Object. Under the stage is placed a 

 mirror, movable in all directions, and which is usually provided 

 with a flat and a concave surface. When it is available, diffuse 

 light never direct sunlight reflected from a white cloud, and a 

 northerly exposure are to be preferred. For ordinary illumination 

 the concave side of the mirror is used. The light is reflected from 

 it, and is transmitted through the hole in the stage, the object on 

 the stage, and the tube of the microscope, to the eye of the 

 observer. The flat mirror is used along with a sub-stage condenser 

 ( 10). 



6. Direct and Oblique Illumination. When the light is re- 

 flected from the concave mirror, it strikes the object nearly verti- 

 cally ; this is called direct or central illumination. But sometimes 

 it is of importance to detect very fine variations on the surface of 

 the object; then for this purpose oblique illumination is practised. 

 This may be done by tilting the mirror slightly, so that the rays 

 of light fall somewhat obliquely on the object. In this case there 

 must be no small diaphragm in or under the stage. This may also 

 be done in the more expensive microscopes by introducing a 

 diaphragm which permits light to pass only at its sides, its 

 centre being blocked. This is known as a central stop-dia- 

 phragm, which shuts off all the axial, and transmits only the; 

 marginal rays, causing what is called dark-ground illumination 



(fig. 12). 



7. The Diaphragm, of which there are two forms in common 

 use. The most common form is a blackened metallic plate dine 

 <Ui>li)'aym perforated with holes of different sizes, placed under 

 the stage, and so arranged as to rotate on a pivot. The edge of the 

 plate usually projects a little beyond the stage, so that it can be 

 readily rotated by the finger, so as to bring the appropriate aperture 

 under the hole in the stage. The diaphragm is usually provided 

 with a slightly projecting pin, which gives a click when the hole in 

 the diaphragm is exactly centred. Another form rylindrn'nl 

 diapliraym consists of a small brass cylinder, into which can be 



