70 ELEMENTARY CHEMICAL MICROSCOPY 



in the negative obtained being the image of one preparation, the 

 other half that of the second preparation. This instrument 

 consists essentially of a stand similar to the Leitz with the 

 microscope tubes joined by a prism chamber and therefore no 

 illustration of its construction is necessary. 



Photomicrographs and polarization studies are of course also 

 possible with the comparison eyepiece described above. 



When two microscopes are available the comparison eyepiece 

 will be found to perform all the work which may be accomplished 

 by means of instruments of the Seibert type and will entail 

 little additional expense to the equipment of the microchemical 

 laboratory. 



Comparison microscopes are indispensable when frequent com- 

 parisons must be made between unknown and known or standard 

 preparations, or when rapid approximate quantitative results 

 are required. In the comparison in different samples of the 

 sizes of fine pigments, of grain sizes in alloys, etc., in the com- 

 parison of different fabrics, etc., etc., this is especially true. 



Special microscopes for micrometric purposes, such as read- 

 ing scales, determinations of the positions of lines in the photo- 

 graphs of spectra, or measuring the diameter of depressions pro- 

 duced in testing for hardness by the Brinell method, will be 

 found described in Chapter VII, page 191; microscopes for the 

 study of ultramicroscopic particles in Chapter V, page 105, while 

 the special types of instrument for the examination of metal- 

 lurgical products and large castings are taken up in detail in 

 Chapter IV. 



For the investigation of molten material, liquid crystals, etc., 

 microscopes of special construction have in recent years been 

 placed upon the market. Most of these have followed the 

 designs of O. Lehmann and comprise a great variety of forms. 1 

 One of the simplest of these is shown in Fig. 29. In this instru- 

 ment polarized light (see Chapter II) is obtained by reflection 

 instead of by the usual manner by means of a Nicol prism, in 

 order to permit swinging the tiny Bunsen burner B below the 

 stage. The light rays reflected from P and R are polarized and 



1 See Lehmann, Das Kristallisationsmikroskop, Braunschweig, 1910. 



