MEDIUM-POWER PH(3T0-MICI10GIIAPHY 



the larger the aperture of the substage condenser, the greater the obliquity that can 

 be obtained. To assist in cutting off extraneous hght, a diaphragm is placed 

 beneath the condenser, between it and the light, having a segmental, crescent, 

 or other-shaped form. Reference has already been made on page 94 to the caution 

 to be exercised in using this form of illumination. It is obtained in some microscopes 

 by reflecting the light obliquely fmm the mirror, which is itself shifted by the 

 operator quite on one side of the optical axis, In the continental form of stand, 



Fig. 58 



where the mirror will not move laterally, but only in all directions about the axis 

 of the microscope, a special arrangement is devised which is shown in Fig. 58, where 

 it IS seen that a small handle e moves the entire mechanism containing the iris (or 

 other super-imposed specially formed diaphragm) out of the axis of the microscope. 

 All that the microscopist has to do then, to obtain oblique light of any extent is 

 simply to turn this handle. But seeing that this movement would only give oblique 

 light in one azimuth, which would be most inconvenient, the difficulty is met by 

 makmg the whole mechanism up to d (the iris diaphragm) revolve completely around 

 the axis of the microscope, which thus enables the operator to obtain oblique light 



