EF, A. Genth—OContributions to Mineralogy. 305 . 
60° prisms, over those of 45°. A much greater angle is objec- 
tionable from the increased distortion produced by the siisblest wf 
imperfection i in the refracting faces. 
risms admitting the same amount of light the more 
acute the angle, the less is the quantity of glass and the less 
the area of each face. The ground might be taken that a 45° 
prism could be made larger than one o at the same ex- 
pense, and thus the difference in light hedials In this case, 
however, it would be necessary to enlarge the telescope, number 
of prisms, and in fact the whole instrument. Even supposing 
this change made, the prisms of larger angle preserve their 
superiority, though not in so marked a degree. The calculation 
is readily made by multiplying the transmitted light by cos 7, 
as 1s done in the above examples, 
The index of refraction varying with a Space! of the 
rays, the oe loss of light, &c., would v: n different 
parts of the spectrum. The change ound, hiseenee,. be small, 
and could be determined, if necessary, = merely altering n. 
Institute of Technology, Boston, Feb. 29th, 186 
Art. XXXIII.—Contributions to Mineralogy.—No. VII; by 
F, A. GEnrTHu. 
Durine the last year or two, I have been engaged with the 
investigation of several very rare minerals, the composition 
of which was but little known, or doubtful, principally from 
localities on the Pacific coast. 
Tn some instances, the results which have been obtained, al- 
though sufficient to establish with great probability the true 
nature of the species, represented, on account of the great 
Scarcity of material, the composition of mixtures of several 
minerals. As for instance with the California tellurids, of 
