362 Wright — Determination of Feldspars. 



and can be prepared by mixing oil of cedar (n na = 1*516) and 

 clove oil (??,„„ = 1*532) for orthoclase and albite; and clove oil 

 (n na =l'532) with cinnamon oil (n lut = l'601) in the requisite 

 proportions for the remaining members. These liquids are 

 miscible in all proportions, evaporate at nearly equal rates and 

 are well adapted for the method. They can be kept in small 

 glass stoppered dropping bottles, mounted on a square wooden 

 block and labelled with the name of the feldspar to which 

 their refractive index corresponds, Another series consisting 

 of monochlorated benzene (1*523), oil of cloves (1*532), aethyl- 

 ene bromide (1*544), nitrobenzene (1*554), mono-bromated ben- 

 zene (1*562), and mixtures of the latter with bromoform (1*595), 

 has been found to give equally good results, but cannot be 

 procured so readily. Dilute Thoulet solutions might also be 

 employed, but are not to be recommended since a slight evapo- 

 ration of the water changes their refractive index rapidly. The 

 solutions can be standardized either by measuring their refract- 

 ive indices directly on a refractometer or by using fragments of 

 typical feldspars as test objects in the method described below. 

 Maniimlation. — In the actual determination of the feldspar, 

 small grains of the substance 0*1 to 0*001 mm in diameter are 

 used and can readily be obtained by breaking up, in an agate 

 mortar, larger fragments of the mineral which have been 

 chipped off the hand specimen. After immersion in a drop of 

 one of the liquids between object glass and cover slip, the 

 grains are observed in obliquely incident light under a micro- 

 scope fitted with a medium power objective and condenser 

 lens slightly lowered.* The nicols should not be crossed. 

 The simplest way to produce oblique rays of light is to cast a 

 shadow on part of the microscopic held by placing the fore- 

 finger between the reflector and lower nicol tube on the 

 microscope. In place of the finger a piece of cardboard or a 

 movable iris or stop diagram can be substituted to advantage.*!* 



* In actual practice, it will be found that the phenomena on which this 

 method depends can be reversed by raising or lowering the condenser lens. 

 If a medium power objective be used, and the condenser lens be raised from 

 its lowest possible position to direct contact with the object glass, it will be 

 noted that the edge of the shadow, cast over part of the field, becomes more 

 distinct on elevation, until at a certain point it is in sharp focus, after which 

 it again becomes less clearly marked. It is in the lower and higher positions 

 of the condenser lens and within the transition zone of the indistinct edge 

 of the shadow that the phenomena are most clearly denned. On passing the 

 focus point, we pass from divergent to convergent rays and the phenomena, 

 which appear, are reversed. In the description below the condenser lens is 

 considered to be in its lower position below the point of sharp focus. As a 

 check on the observations, it is often advisable to raise the condenser lens 

 and observe the reversed phenomena. By placing the condenser lens in 

 suitable position, the highest power objectives can be used equally well 

 and minute particles thus be studied. 



f Compare Tscherm. Min. Petr. Mittheil., xx, 238-239. 



