Mr. William Phillips on the OxydofTin, 347 



sonal instructions from the ingenious and scientific inventor. Be- 

 fore Ihad arrived at some tolerable knowledge in its use, so as to be 

 assured that the smaller crystals only can be relied on, the great 

 differences which I found to exist in the same angles of the larger 

 crystals, even though their planes appeared by the assistance of the 

 magnifying glass, to be undeviating and polished surfaces, almost 

 tempted me to doubt the utility of the instrument itself. These 

 differences amounted in many instances to as much as 15', fre- 

 quently 10' ; while on the other hand, small crystals, having clear 

 and perfect reflections, gave a coincidence in the admeasurement 

 of the same angle.* 



I feel therefore warranted in the conclusion that, although occa- 



* The reflecting goniometer is so delicate an instrument, that great care is requisite in 

 the choice of the crystals subjected to it for the admeasurement of their angles. It 

 often happens that those of apparently the most beautiful surfaces are unfit for this 

 purpose; the most clear reflections alone can be relied on, and even then only by com- 

 paring the results of trials on many crystals. Some of the first attempts gave an inci- 

 dence of 2 on 2 over the apex of the fig. 27. PI. 16. one v.ay of 92°. 35'. the other way 

 93°. 20'. or even 93°. 25'. and this induced the suspicioa that the bases of the twa 

 pyramids composing the primitive octohedron, were not square. The crystals on which 

 those admeasurements were taken, were, comparatively, large, and their reflections were 

 by no means so clear as those since obtained on much smaller ones, which have con- 

 firmed the real incidence both ways to be 92°. 55'. and therefore that the common base 

 of the two pyramids is square. 



The crystals of this substance are likewise subject to another diflicuUy, that of a 

 double reflection, even on faces which, by the assistance of the lens, appear of the 

 most perfect kind. I possess a crystal giving two reflections on three of tiie four faces, 

 2, 2 fig. 27. which are those of the pyramid commonly observed on the crystals of this 

 substance. The incidence obtained one way over the apex, with the two strongest re- 

 flections was, 92°. 55'. with the two weaker 93°. 10'. but with a strong reflection on 

 gne face and a weaker on the other 93°. 5'. On one of the other two opposed faces of 

 the pyramid, one reflection only was given, but on the other, two were visible; with 

 the strongest reflection, the incidence obtained was 93°. 35'. with the other, 93°. 25'. 

 the least of them 30'. above the real value of the angle, 



2x2 



