l86 ELEMENTARY CHEMICAL MICROSCOPY 



Contour bands appear whether the solid has either a higher or 

 a lower index than the surrounding liquid. The next step must 

 therefore be to ascertain whether it is higher or lower than the 

 liquid employed.' This is accomplished by slowly raising the 

 microscope tube by means of the coarse adjustment, at the same 

 time closely observing the change in appearance, direction of 

 motion or change in color of the contour bands and halo-like band 

 of light bounding the crystal fragments. When the solid pos- 

 sesses a higher index than that of the liquid, the contours are 

 usually dark and well defined with a halo or band of light within 

 the black bands; as the microscope tube is raised this band of 

 light will appear to move inward, i.e., toward the center of the solid. 

 If, on the other hand, the solid possesses a lower index of re- 

 fraction, the black contours are relatively weak, with the bright 

 halo outside the black bands, and upon raising the objective the 

 band of light or bright halo appears to move outward or away from 

 the center. This difference in behavior is due to the fact that 

 when the fragment has a higher refractive index than the liquid 

 it causes the rays leaving it to converge, but if the solid has a 

 lower refractive index the emerging rays are divergent. In order 

 to obtain the best results by this method, always screen the prepa- 

 ration upon the stage with the hand ; thus none but transmitted 

 light rays can enter the objective. 



By employing oblique instead of axial light it becomes still 

 easier 1 to determine whether the solid possesses a higher or a 

 lower refractive index than the liquid in which it is immersed. 



Before considering the method of procedure in this case let us 

 study several simple yet instructive experiments. 2 



Place a small drop of mucilage or thin gum upon an object 

 slide, beat it with a knife blade until full of air bubbles. Cover 

 with a cover glass and place upon the stage of the microscope. 

 Use an 8 millimeter objective and center a tiny air bubble whose 

 image appears to be not over i to 2 millimeters in diameter. 

 Focus sharply. The image obtained will consist of a tiny disk 

 of light surrounded by a black ring. We are here dealing with a 



1 Schroeder van der Kolk, Zeit. anal. Chem., 38 (1899), 615. 



2 Gage, The Microscope (1908), p. 106, loth ed., Ithaca, N. Y. 



