The Microscope. 175 



quartz sections, at most .45 mm. It is difficult to get colors with 

 sections of Iceland spar itself, because the thickness of the sec- 

 tion must not exceed the one-fortieth part of a millimeter. 



Thus, while we have in the microscope itself an instrument 

 for revealing a wonderful variety and beauty of form hidden to 

 the naked eye, the polariscope reveals matchless beauty of color, 

 of which no microscopist, no teacher of the youth, in fact no 

 -one, can afford to be ignorant. These phenomena are of sur- 

 passing interest in themselves. And the chief object in writing 

 this paper is to direct to them the attention of all concerned. It 

 would lead into deep water, and would be far from interesting to 

 treat here more fully of the laws of Crystallization, the optical 

 properties of crystals, the intimate relation of the force of crys- 

 tallization and of light as suggested by the phenomena of the 

 polariscope, unless the reader be first familiar with the facts. 

 Those interested will naturally go to the higher works on this 

 subject. The writer has access only to Deschanell's Natural 

 Philosophy, and to a French work by'Guillemin, " Les Phenome- 

 nes de la Physique," both very helpful to a good understanding 

 of this subject. But first of all — facts ; let every one who can 

 4ifford it get a polariscope and set about seeing these things for 

 himself 



Finally, a word about the practical uses of the polariscope. So 

 far it has figured as only an exquisite toy, or by suggestion at 

 most as an aid in unravelling the mysterious nature of light and 

 its relation to chemical force and cristallization. Perhaps in 

 this same category may be placed the first use here to be men- 

 tioned, namely that of determining whether bodies like the 

 planets and the moon shine by reflected light or are self-lumin- 

 ous. For reflected light, being polarized, if examined through 

 a Nicol prism, will darken the field of view at two points 180° 

 apart. Then the play of colors in certain of the plant and ani- 

 mal tissues, show that the cell walls in these tissues are doubly 

 refractive. As Arago has shown, the jeweler may use the pol- 

 ariscope as a test of precious stones, by measuring the polarizing 

 angle on one of the ground faces ; for instance, that of the diamond 

 is 67°-70°, of flint glass, 57°-58°. So also, in crystallography in 

 general, the polariscope enables us to distinguish crystals with 

 one or two axes and to determine their position. Further, in the 

 investigation of sugar adulteration, or saccharimetry, this instru- 



