Elementary chemical microscopy elementarychemi00cham Year: 1921 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 satisf
Elementary chemical microscopy elementarychemi00cham Year: 1921 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 fas, 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- 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 forme
Size: 1655px × 1208px
Photo credit: © Bookend / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: archive, book, drawing, historical, history, illustration, image, page, picture, print, reference, vintage
