32 HOW TO USE THE MICROSCOPE 



piece of camphor in the oil. The hght from the 

 lamp is concentrated by placing a cylinder of white 

 cardboard over it. This cjdinder must, of course, 

 be open at the top for the escape of heat, and have 

 air-holes at the bottom ; in addition, a suitable 

 opening through which light can proceed from the 

 flame to the mirror is needed in the side. A metal 

 reflector is not advantageous. 



The lamp should be placed about a foot from the 

 mirror, and when low powers are used the fiat of 

 the flame is turned to the mirror ; but with the 

 ^-inch and higher powers the edge gives more satis- 

 factory results. 



Most microscopes are fitted with diaphragms 

 beneath the stage. In the older stands they usually 

 consist of rotating discs of metal pierced with holes 

 of varying diameters. In more modern stands they 

 take the " Iris " form, which can be regulated to 

 any aperture. The diaphragm is used to moderate 

 illumination of the object by cutting off unnecessary 

 rays of light. The worker will soon learn to use 

 the diaphragm with judgment, taking care to use 

 only such light as is necessary. His concern will 

 be to concentrate an even light upon the object, and 

 to avoid dazzling his eye with a flood of light ex- 

 tended beyond the object for which there is no prac- 

 tical use. When objects are so dazzlingly illumin- 

 ated that the eye is rendered uncomfortable, the 

 light is easily toned by placing a slip of blue-tinted 

 glass under the object. 



Up to the present point I have had in mind only 

 such objects as can be examined by transmitted 

 light, and these will be found in the majority. But 



