ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 39 



formed in the field of the microscope and the true structure of 

 the object at this point will prove very difficult of interpretation. 

 When, however, axial reflected light is used, that is, when the 

 illuminating beam strikes the polished preparation normal to 

 its surface, the plane surfaces will appear bright, the irregularities 



FIG. 12. Path of Oblique Light Rays 

 striking an Irregular Surface. 



FIG. 13. Path of Axial Light Rays 

 striking an Irregular Surface. 



more or less dark, and minute projecting irregular points will 

 yield diffraction patterns; for as shown in Fig. 13, the light 

 rays b and c, striking reflecting surfaces, are turned aside at such 

 an angle as to preclude their entering the objective. 



Careful consideration of the above described phenomena is 

 absolutely essential to a correct interpretation of the structure 

 of the material being studied. To determine when one is dealing 

 with depressions and when with elevations when working with 

 moderately high powers and vertical or oblique illumination is 

 often a difficult problem which is further complicated for the 

 beginner by the fact that the image seen is that of the object in 

 a completely reversed position. 



It is obvious that the oblique illumination of opaque objects 

 is restricted to low powers, since the free working distance of 

 high power objectives is so small that the path of any pencil of 

 light which will strike the preparation at a point lying in the line 

 of the optic axis of the microscope must then be so oblique as 

 to be approximately parallel to the surface of the preparation. 



Light rays reflected from the surfaces of anisotropic crystals 

 are polarized, but are not noticeably polarized if from iso tropic 

 crystals. It therefore often proves of great value in qualitative 

 analysis to employ polarized light for the illumination of objects 

 to be studied by means of vertical illuminators. 



