418 F. K Wright — New Ocular for Use with the 



path difference between the emergent light waves. The thick- 

 ness of the plate can be measured either bj direct contact by 

 micrometer screw or spherometer or by means of the fine 

 adjustment screw of the microscope, or indirectly by means of 

 the interference color or path difference of an adjacent mineral, 

 properly cut and of known birefringence. Of these different 

 methods the second is most convenient, although possibly less 

 accurate. The usual method consists in bringing to sharp 

 focus the upper surface of the plate and then the lower surface 

 as seen through the plate itself, or if the plate be free along 

 one side, to focus on the object plate on which the section is 

 resting. In the first case, the apparent thickness must be 

 multiplied by the refractive index of the mineral to obtain the 

 true thickness. Since the average thickness of minerals in the 

 thin section is about -030 mm , an error of *001 ram in setting 

 the micrometer screw will produce an error of 3 per cent in 

 the thickness determination. In ordinary microscopes this 

 error may frequently amount to '002 or -003, and the resulting 

 error in thickness to 10 per cent. 



Suppose the extreme limits of error be •003 ,Qm or 'OOIS 111111 on 

 either side of the true value, then an error of 5 per cent in the 

 actual thickness determination may be considered probable. 

 If this probable error be increased to 8 per cent to allow for 

 multiplication by the refractive index and to introduce a safety 

 factor, it can safely be assumed that the thickness of favorable 

 crystal plates in thin sections, ranging from *03 to -05 mm in 

 thickness, can be determined readily by this method within 8 

 per cent of the true value. For minerals in powder form, the 

 thickness of the individual grains may be much greater and 

 the thickness determination correspondingly more accurate. 



On the wedge whose scale divisions correspond to 10 /jl/h 

 path difference of light waves, the error of determination is 

 not over one division on the scale (*l mm ), which is less than 2 

 per cent. 



The total probable error of the determination of the bire- 

 fringence of a mineral plate in the thin section in an unfavor- 

 able instance may amount, therefore, to 10 per cent. As the 

 birefringence of the ordinary rock-making minerals ranges 

 from about "005 to '050, an error of 10 per cent is confined to 

 the third decimal place.* 



In determining the birefringences (y-a) or (y-/3) or (/3-a) of 

 a mineral the position of the mineral plate (under examination) 



* An average of the birefringences of the 118 minerals listed under bire- 

 fringence on page 292-295 of Rosenbusch-Wiilfing, Micros. Phys. I, 1, 

 gives '040 as the mean value, while the value of the members midway 

 between the two limits is "020- "025. This value represents more nearly 

 the mean value of the birefringence of rock -making minerals than the arith- 

 metical mean, "040. 



