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 Hght 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 pohshed 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 
ah, cd and ef can enter the objective to 
form an image since the angle of reflec¬ 
tion is equal to the angle of incidence. 
The surface will therefore appear dark. 
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 held of the microscope and the true structure of the ob¬ 
ject at this point will prove very difficult of interpretation. 
Fig. 6. Path of Oblique 
Light Rays striking a 
Plane Polished Surface. 
