30 ELEMENTARY CHEMICAL MICROSCOPY 



centrating the light rays into a bright spot of light upon the sur- 

 face of the object at a point lying approximately 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 polished specimen, simply holding it slightly inclined upon 

 the stage will send sufficient light into the instrument to permit 

 a thoroughly 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. 

 6 (as, for example, a polished but unetched metallurgical speci- 

 men), lying in a plane normal to the 

 optic axis of the microscope, and we 

 illuminate 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 reflec- 

 FIG. 6. Path of Oblique t i on j s equal to the angle of incidence. 

 p! ght P 71 T* [ 7 a The surface will therefore appear dark. 



Plane Polished Surface. 



The more nearly a perfect 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 practically 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 diagrammed in Fig. 7, 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 formed 

 in the field of the microscope and the true structure of the ob- 

 ject at this point will prove very difficult of interpretation. 



sV 



