204 



LECTURES TO SCIENCE TEACHERS. 



forming some general idea of illumination, I think we may 

 learn a great deal by studying what we see in small crystals, 

 and in inorganic bodies of pretty well known form. If light 

 be sent straight up through a small crystal, having parallel 

 faces terminated by an oblique plane, it will pass up directly 

 in one part and be bent .and thrown aside altogether in the 

 other, and in looking at such a crystal with the microscope, 

 you might see a black edge with perfect definition, but if you 

 were to throw the light obliquely with a condensing lens of 

 considerable aperture, the light might enter at such an angle 

 that it would be bent and pass straight out, and if the edge 

 of the crystal were absolutely perfect you would see no dark 

 band at all. When thus illuminated, the crystal might be 

 quite invisible. 



Ft G./. 



FIG. 2. 



Ml 



My meaning will be better understood by means of the 

 following rough illustrations. 



In Fig. 1 the light is supposed to be all parallel, and is 

 bent quite out of the line of vision by the oblique end of the 

 crystal, whereas in Fig. 2 the light is supposed to be so very 

 variably divergent, that some of it can enter the object-glass 

 after passing through the crystal, both where the sides are 

 parallel and where they are inclined. Such a case clearly 



