IDENTIFICATION OF FERTILIZER MATERIALS. \) 



the optical character of a biaxial crystal can be determined when 

 only one hyperbola appears in the field. The phenomena are similar 

 to those presented when both hyperbolas appear. 



OPTIC AXIAL ANGLES. 



In the interference figure of sections cut normal to the acute 

 bisectrix and rotated into the 45° position, the distance between the 

 hyperbolas, i. e., between the two dark spots, represents the angle 

 between the two optic axes. The observed angle, 2E, however, is 

 not identical with the actual angle, 2V. The symbol 2H is some- 

 times used to represent the angle when it is measured in an oil. The 

 distance between the points of emergence of the axes, in the 45° 

 position, can be measured by a micrometer eyepiece. Let this dis- 

 tance be 2d. Then 



SinE =£!• 



E is half the axial angle in air, d is half the distance between the 

 points of emergence of the optic axes as measured with the eyepiece 

 micrometer, and C is a constant which must be determined for each 

 lens combination. It may be determined by using a mica plate, 

 whose axial angle is known, as an object and substituting in the 

 formula. If n is the mean index of refraction of the mineral, then 



SinE 

 SniV=— — ■ 



2V can thus be readily found. In a large number of cases the simple 

 designation of the angle as " large" or " small" will serve all purposes; 

 and this relative size may be observed by a glance at the interference 

 figure. 



DISPERSION. 



Here p is used to indicate red light, and o violet light. Observa- 

 tions are made on the interference figures. 



In the orthorhombic system the dispersion is least, p<v or p>\, 

 for the color within the circle which is nearest the bisectrix and which 

 touches the concave side of the hyperbola when the crystal is in the 

 45° position. The colors are symmetrical to the bisectrix. 



In the monoclinic system there are three kinds of dispersion, 

 inclined, horizontal, and crossed. -In inclined dispersion the colors 

 are arranged symmetrically to the line joining the loci of the hyper- 

 bolas, but are not symmetrical in the other direction. One locus is 

 oval shaped, larger, and less intense ; and the other is round, intense, 

 and smaller. In horizontal dispersion, the colors are not arranged 

 symmetrically to the line joining the loci of the hyporbolse. They 

 are symmetrical, however, to a line at right angles to this line. In 

 crossed dispersion there is only a point of symmetry. 



