SPECIFYING QUARTZ CRYSTAL ORIENTATION 257 



can be used to judge the difference between the Hquid index and the quartz 

 ordinary index. 



The Hquid can be adjusted by placing a cap face of a crystal on the refer- 

 ence glass and setting the dial to read 51.8° the liquid being then blended 

 to center the pattern. 



If the refractive index of the fluid is low by an amount L the observed 

 reading R must be corrected by adding to it an amount e. 



Since 



{Ng - L) sinR = N sin (R-e) 



we can compute e. The accompanying refraction correction nomograph 

 was computed from the above equation. If we know that the fluid in the 

 conoscope is high, by an amount //, and we wish to know the correction to 

 be applied to a conoscope reading R we locate H on the diagonal line HL 

 and locate R on the horizontal line R. We join these two points with a 

 straight line and read the scale e—e where this straight line crosses the 

 curved line e—e. This value tells the size of the correction and whether 

 to add it to R or subtract it from R. Conversely, if we wish to find how 

 closely the index of the fluid must be held in order to have the correction 

 less than say j° at a reading of say R = 50° we join the points R = 50° and 

 e — dz j° and find H = dc 0.005. A ten-inch-long Incite strip with a 

 straight line ruled on it is a convenient tool with which to read this nomo- 

 graph. 



We now inquire as to whether the refraction correction can be made to 

 annul the ring eccentricity correction. In the appendix it is shown that 

 this is done ii H = — .530 tan- d where 2d is the distance between the ver- 

 ,tical reticule lines. 



Experimentally it is easy to achieve this balance by using a cap face 

 parallel slice. With the cap face against the reference glass and the dial 

 reading 51.8° the fluid is blended to make a single ring tangent to both reti- 

 cule lines. When this is done for d = 10° the fluid should have a refractive 

 index of 1.5228 and the residual errors should be less than 2 minutes for R 

 not over 60°. 



2.9 Immersion Fluids 



In order to match the refractive index of quartz we blend a substance 

 which has an index that is too large with one that has an index that is too 

 small. Such blended substances should be liquid at room temperature and 

 hence should be perfectly mutually soluble. They should have low vapor 

 pressure so that they do not evaporate quickly and should be harmless to 

 the operator. Also they should be nearly colorless and clear. They should 

 be fluent enough to be easily drained from the crystal and should have a 



