2 BULLETIN 97, U. S. DEPARTMENT OF AGRICULTURE. 



It was found that potassium chloride, potassium sulphate, ammo- 

 nium sulphate, kainite, sodium nitrate, calcium nitrate, apatite, 

 gypsum, limestone, and other ground rocks and minerals could be 

 identified definitely by microscopic-petrographic methods. For the 

 equipment and modus operandi of determining substances by their 

 optical characters reference is made to the literature of the subject. 1 

 A brief outline only is here given. 



EQUIPMENT. 



For the determination of the optical constants of the various salts 

 and consequently for the identification of the salts themselves, a 

 petrographical microscope is practically a necessity. Ordinary 

 microscopes converted into petrographic microscopes by the addi- 

 tion of various adjuncts are clumsy at best and are far from being 

 satisfactory. Petrographic microscopes, as at present manufactured, 

 have reached a high degree of perfection and, considering the quality 

 of the workmanship and their all-round usefulness, the price is not 

 exorbitant. For a full description of these instruments, reference is 

 made to the trade catalogues. A few words concerning them, how- 

 ever, are necessary in order to render intelligible the description of 

 the manipulations used in determining the optic constants. 



The petrographic microscope is, in general make-up, similar to 

 ordinary microscopes. It has both coarse and fine adjustments for 

 focusing; the stage is constructed so as to revolve around the axis of 

 the instrument and is graduated into 360°, so that the angle of any 

 revolution may be read off directly. Just below the stage is a con- 

 denser lens so fitted that it may be readily thrown in or out at pleasure. 

 Below the condenser lens is a nicol prism which acts as a polarizer, and 

 below the polarizer is the mirror. The objectives are attached to the 

 tube of the microscope by a clamp device which admits of their 

 ready insertion or removal. The objective is centered by two screws 

 acting at right angles to each other. In the tube of the microscope 

 is another nicol prism, the analyzer, which may be thrown in or 

 out of the line of vision and which may be rotated from a 

 position parallel to the polarizer to a position at right angles to it. 

 Between the objective and the analyzer is a slit at an angle of 45° to 

 the planes of the nicols through which various adjuncts are inserted 

 as needed. Above the analyzer is another slit through which another 

 accessory, Bertrand lens, is inserted as needed. The oculars have 

 cross hairs at right angles to each other and parallel to the planes of 

 the nicols. 



i Bul.91, Bureau of Soils, U.S. Dept. Agr. Iddings, Rock Minerals. Johannsen, Determination of Rock- 

 Terming Minerals. Rosenbusch and W tilting, Mikroskopische Physiographic der petrographisch wichti- 

 gen Mineralien, 2 vols. 



