THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 625 



the *' universal " or theodolite stage by means of which the 

 properties of light vibrating in different directions may be 

 studied in parallel light in the object- image of a single section 

 by rotating the latter on two or more axes. The subject is, 

 however, too extensive to be considered on this occasion. 



Other Determinations. 



The Thickness of the Rock-slice. The only practicable method 

 of determining the thickness of an ordinary rock-slice is to select 

 a known mineral whose maximum birefringence is practically 

 constant and not too low, such for instance as quartz, orthoclase, 

 olivine, calcite and (for approximate results) an acid or inter- 

 mediate plagioclase. Search is then made for the section of this 

 mineral which shows the highest relative retardation, and it 

 may be assumed that its birefringence has as nearly as possible 

 the maximum value. Suppose the mineral to be quartz, with 

 a birefringence of 9 millesims (0'009), and the greatest relative 

 retardation observed to be 315 micro-millimetres. Then the 

 thickness of the section will be 315 -f- 9 = 35 microns. 



The thickness should be determined, if possible, at several 

 points so as to obtain an idea of its variation in different parts of 

 the rock-slice. If the thickness of the rock-slice is not uniform, 

 that of the crystal section must be estimated from its position 

 in the slice as nearly as possible. The thickness should be stated 

 on the sketch, and indicated by the depth of the scale (fig. 1). 

 ]f the thickness is not uniform, the amount of variation may 

 be indicated in the same way. 



Determination of the Birefringence. Knowing the thickness of 

 a crystal section and its relative retardation, we are able to 

 determine its birefringence by dividing the latter by the former. 

 For instance, if the section has a relative retardation of 340 

 micro-millimetres and a thickness of 28 microns, the birefringence 

 will amount to 340 -j- 28 = 12 millesims or 0*012. 



A number of different crystals of the same mineral are dealt 

 with in this matter, and it may be assumed that the maximum 

 birefringence thus obtained falls but little short of the maximum 

 birefringence of the mineral. 



The Refractive Index. A knowledge of the index of refraction 

 of a mineral is a valuable means of recognition. In the case 



Journ. Q. M. C, Series IT. No. 77. 35 



