116 W. O. Crosby—Pinite in Eastern Massachusetts. 
objective to not more than three; we are now in a position to 
appreciate the absurdity of my critic’s assertion, page 429, 
when, enquiring if it is possible to eradicate the secondary 
spectrum by increasing the number of lenses in an objective, 
he says, ‘Theoretically, since a new disposable constant for 
color change is introduced with each lens in the system, the 
answer is evidently affirmative; * * *” 
For an objective consisting of more than two lenses, and a 
law of dispersion involving more than two powers of the wave- 
length, the condition given in my former paper, page 196, for 
the best color correction, is no longer applicable. The problem 
then becomes very complex, but I am inclined to think that it is 
satisfactorily solved by attributing to each element of the focal 
curve a mass proportional to its efficiency for the purpose for 
which the correction is required, and varying the curve until 
its moment of inertia about its intersection with the focal plane 
becomes a minimum. It is also probable that this condition 
will suffice to determine the relative merits of double and triple 
Washington, Dec. 29, 1879. 
Art. XIV.—Pinite in Hastern Massachusetts: its Origin and 
Geological Relations ; by W. O. CRossy. 
ONE of the most interesting constituents of the conglomerate 
so extensively developed in the vicinity of Boston is a 80 
greenish and somewhat unctuous, amorphous mineral, which 
many observers have mistaken for serpentine, but which 18 
shown by its ready fusibility not to be magnesian ; while analy- 
sis proves that it is essentially a hydrous alkaline silicate of 
aluminum. In fact, it presents in its chemical, as well as 1ts 
physical, characters a close agreement with the species pinite. 
(See analyses below). The hardness is ordinarily near, or 4 
little above, 8; the purer varieties, however, usually refuse to 
scratch calcite. The specific gravity, so far as determined, 18 
between 2°7 and 2°75. Luster none, or waxy and feebly shin- 
ing. The predominant color is a whitish-green; but the varia 
tion is from nearly white through whitish, grayish and dirty 
