38 ELEMENTARY CHEMICAL MICROSCOPY 



fleeting surface is mounted in a cell attached to the microscope 

 just above the objective. In these devices the reflector sends 

 the illuminating beam of light through the objective which acts 

 as the condenser, concentrating the light rays into a bright spot 

 of light upon the surface of the object at a point lying approxi- 

 mately in the optic axis of the microscope. From the surface 

 of the object the rays are reflected back through the objective 

 and form the image of the object in the usual manner. 



When only very low powers are required for the examination 

 of a specimen, simply holding it slightly inclined upon the stage 

 will send sufficient light into the instrument to permit a thor- 

 oughly satisfactory study of the coarse details. Slight focusing 

 up and down will answer all purposes. 



Since reflected axial and oblique light must very frequently 

 be employed by the chemist it is essential that he should thor- 

 oughly understand the phenomena exhibited by different sur- 

 faces illuminated in different ways. 



If we are dealing with a highly polished mirror surface S, 

 Fig. ii (as, for example, a polished but unetched metallurgical 

 specimen), lying in a plane normal to the 

 optic axis of the microscope, and we il- 

 luminate it by reflected light, it is obvious 

 that none of the oblique rays ab, cd and 

 ef can enter the objective to form an image 

 since the angle of reflection is equal to the 

 FIG. ii. Path of Oblique angle of incidence. The surface will there- 

 Light Rays striking a fore appear fo,^ Tne more nearly a per- 



Plane Polished Surface. , J F 



feet reflecting surface the object possesses, 



the darker it will appear. It will remain dark until the ray ef 

 becomes almost parallel to the optic axis and therefore prac- 

 tically normal to the surface of S. Reflected light rays now 

 can enter the objective and the surface appears bright and shining. 

 But if the surface of the object illuminated by the oblique 

 rays is irregular or etched, as diagramed in Fig. 12, then the 

 irregularities will appear bright, the plane or polished surfaces 

 dark. If a light ray a strikes a series of tiny minute points as 

 at D, the light will be diffracted; diffraction patterns will be 



