U. S. DEPAETMENT OF AGEICULTUEE. 
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. 
1 Bui. 91, Bureau of Soils, U.S. Dept.Agr. Iddings, Rock Minerals. Johannsen, Determination of Rock- 
Forming Minerals. Rosenbusch and Wulfing, Mikroskopische Physiographie der petrographisch wichti- 
gen Mineralien, 2 rols. 
