118 PRACTICAL DIRECTIONS 



A little xylol can also be used to clean the stage. 



The coarse and fine adjustments should be lubricated from time to time by 

 the application of a trace of vaseline. 



SECTION IV. METHOD OF USING THE MICROSCOPE. 

 1. The source of light. 



The microscope when in use should rest on a firm table in front of a window. 

 The best light for microscope work is that reflected from a white cloud, but 

 the light may be taken also from a clear sky or a white wall. Direct sunlight 

 is totally unsuitable. 



In default of a satisfactory natural light, a good petrol-air or albo-carbon 

 lamp may be used, though a lamp such as Ranvier's with an Auer burner is 

 better. With these lamps it is sometimes necessary to interpose a sheet of 

 ground glass between the source of light and the microscope to moderate the 

 intensity of the former. 



[Many observers prefer a small oil lamp, but for general use a very satis- 

 factory artificial light is to be obtained by the use of an inverted incandescent 

 gas mantle, the light from which is passed through a large flask filled with 

 distilled water before reaching the mirror (fig. 111).] 



FlQ. 111. Illumination with an inverted incandescent gas burner. 



Turn the microscope towards the source of light, look down the tube, and 

 taking hold of the sides of the mirror move the latter about until the field 

 is brightly illuminated. 



1. With dry lenses use a concave mirror, which throws a convergent pencil 

 of light on to the object. 



2. When using an immersion lens, it is necessary to have an Abbe condenser 

 fitted below the stage. With a condenser aflat mirror must always be employed ; 

 the rays reflected from the flat mirror are converged by the condenser and 

 brought to a focus on the object, so that by the use of a condenser a con- 

 siderable amount of light is obtained. 



Every microscope should be provided with a diaphragm below the stage. 

 The size of the opening in the diaphragm will be determined by the magnifi- 

 cation employed ; the greater the magnification the smaller should be the 

 opening in the diaphragm. By cutting off the marginal rays which are 

 not only useless but actually detract from the sharpness of the image the 

 diaphragm assists in the correction of spherical aberration, and produces a 

 sharper definition of the object. 



