ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 21 



without moderately high powers and some form of substage 

 condenser. It is therefore a safe rule to always employ a sub- 

 stage condenser unless exceptionally low powers are to be used; 

 this of course does not apply to problems involving examinations 

 with polarized light. 



All modern compound microscopes are provided with two 

 mirrors placed back to back in an annular mounting. One 

 mirror has a plane surface, the other a concave surface. The 

 mounting is so pivoted as to permit the easy swing of one or 

 the other of the mirrors into a position to reflect light through 

 the stage opening. The plane mirror is employed with day- 

 light or light diffused from a ground glass placed before an arti- 

 ficial light. With the plane mirror parallel light is obtained. 

 The concave mirror serves to obtain parallel rays from a source 

 of artificial light placed only a short distance from the mirror or 

 may be employed as a collector of rays (converging light) when 

 powerful illumination of the object is desired. Microscopes in 

 which the linear distance between mirror and stage is fixed 

 and unalterable should be provided with diaphragms to fit 

 immediately below the object. If no such device is provided, 

 it will be found desirable to have at hand several pieces of dull 

 black paper or thin card through which have been cut circular 

 orifices of different diameters. Unless diaphragms are used 

 below the object details of fine structure can rarely be discerned. 



b. Transmitted Oblique Light is essential for the proper inter- 

 pretation of appearances under the microscope of objects whose 

 upper and lower surfaces are so placed as to lead to serious 

 confusion if axial light is alone employed. Oblique light also 

 aids in establishing whether the liquid medium or the object 

 immersed in it has the higher refractive index. The value of 

 oblique illumination may be better understood by referring to 

 the diagram shown in Fig. 3. A transparent object O whose 

 upper and lower surfaces are identical and perfectly symmetrical 

 is shown in section, lying upon an object slide upon the stage, 

 with perfectly axial light as shown by the arrows. It will be 

 obvious that even very careful focusing will fail to disclose the 

 probable structure of the lower surface and that even the upper 



