192 USE OF THE MICROSCOPE. 



stationary. But if the mirror should not be employed, and 

 the light obtained direct from the lamp, then, in order to 

 throw the image of the lamp in the middle of the illuminated 

 disc, the adjustment must be made by a lateral movement 

 of the whole microscope, and by varying its inclination ; the 

 image being thus found, the lamp and microscope must remain 

 stationary. The object-glass used in this preliminary adjust- 

 ment is to be removed, and the object-glass to be employed 

 for observation is then to be screwed on to the microscope 

 tube in its place, and the object brought into focus, which 

 will also bring the image of the flame of the lamp distinct ; 

 but the distinctness, as also the centricity of the image of the 

 flame, may have been in a degree deranged by shght difier- 

 ences in the screws of the object-glass and other minute 

 circumstances. This deviation is to be rectified by moving the 

 microscope tube sideways and back again across the image of the 

 flame, and, if adjustment be necessary, by turning the mUled- 

 head screw which presses against the front edge of the stage, 

 as before described, while the mirror and lamp remain sta- 

 tionary. Slight obUquity of the illumination subdues the 

 glare attendant upon perfectly central and full illumination by 

 lamp-light; and this obliquity may be obtained by slightly 

 altering the position of the mirror; or if the mirror is not 

 employed, but light is obtained by pointing the microscope 

 tube directly to the lamp, then the obliquity required may be 

 obtained by a small variation of the inchnation of the micro- 

 scope." 



If a stop with a smaU. aperture be placed temporarily in the 

 lower end of the condenser, so as to render the spot of light 

 smaller than the field of the eye-piece, it will not be necessary 

 to put on a low power object-glass to make the adjust- 

 ments. 



Some achromatic condensers are provided with a prism of 

 the form represented by fig. 63, as a substitute for the plane 

 mirror. M. Dujardin employed it with his condenser, and the 

 first of these instruments brought to this country was supplied 

 with one. The method of using it differs so little from that 

 of the ordinary mirror, that no especial directions need be given. 



