Till: MEASUREMENT <>r ruvsTALS 



that any one below may be subtracted directly from any one 



above it. 



In the prism zone on the crystals of topaz from Thomas Moun- 

 tain- there ;ire usually two pri-m- :tnd the bradiypinacoid, yield- 

 ing ten readings to complete the 360. Fig. 305 is a plan of this 

 /one with the faces lettered according to the usual practice. Let 

 the first reading l>e taken of the face m. With the edge m*! ad- 

 justed the signal from m is brought to the vertical hair as described, 

 then a reading of the vernier is taken with a lens, and the number 

 of degrees and minutes recorded in the notes opposite the letter 

 standing for the face on the 

 sketch. All faces of the zone 

 are measured in the same 

 manner, and as their inter- 

 secting edges are parallel, 

 but little adjustment should 

 be necessary as each edge in 

 order is brought to the ver- 

 tical hair. It is usual in 

 accurate work to make three 

 readings of the same angle, 

 using different parts of the 



graduated circle each time, to avoid being influenced by the same 

 number ; and the average of these three results is taken as being 

 more nearly correct than any one. It is also well to note opposite 

 each reading the character of the signal reflected by the face, as to 

 whether it is well defined and bright, or irregular, dull, diffused, or 

 complex from striations, as well-defined and bright signals will 

 yield results nearer the truth than any poor signal will and all 

 readings from bright sharp signals are to be given more weight in 

 results. The following are the results of the readings in the prism 

 zone of such a crystal of topaz : 



FIG. 305. Section of a Topaz Crystal 90 to 

 the Prism Zone. 



m 

 1 

 b 

 I' 



READINGS 



= 358 42' 



= 339 55' 



= 296 33' 



= 253 04 



ANGLES 



b A m 

 b.m' 

 1*1' 



= 62 C 

 = 62' 

 = 86 C 



9';b 

 9';b 

 51' 



, 1 = 43 22' 

 J' = 4329' 



m' = 234 24' 

 m" = 178 42' 

 1" = 159 52' 



m A m' = 124 18' 

 m' A m" =55 42' 

 l' A l" =93 12' 



