DALY. — AMPHIBOLES AND PYROXENES. 



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tually trying certain cases, I have found that p changes for small errors 

 in 6' and $", but that the rate of change is not sufficiently rapid to make 

 the method of no practical use. Indeed, the errors in p may in certain 

 cases be little greater than the original errors of reading 6' and 6". A 

 Zillerthal actinolite may be taken as an example. 



Let C = 62° 15', and 2 r=: 80° (an actinolite). Then 6' = 13° 27'; 

 and, if C" — 77° 15' (15° of rotation of the vertical circle), then 6" = 

 8° 16'. In the attempt to find p from 0' and 6", errors of various 

 magnitudes in 0' and 0" were introduced as follows. 



TABLE in. 



The limit of error in reading extinction-angles for colorless cleavage 

 flakes, or for those that do not display very deep absorption, may be 

 placed at 20'. At this limit, in our example, there is seen to be an error 

 of rather less than 1° for an error in 0", with 0' exactly determined. 

 When 6" and 6" show errors of reading in the same direction (in the 

 example, both plus), they may be large without making an equally large 

 error in p. This is important, since any fault in the construction of 

 the microscope will thus equally affect both readings and need not greatly 

 influence the value of p, even though found by means of so sensitive an 



