214 £.. 8. Dana—Stibnite from Japan. 
Art. XXIV.—On Mr. Glazebrook’s Paper on the Aberration of 
Concave Gratings ; by H. A. Rownanp. 
In the June number of the Philosophical Magazine, Mr. R. T. 
Glazebrook has considered the aberration of the concave grat- 
ing and arrives at the conclusion that the ones which I have 
hitherto made are too wide for their radius of curvature. As 
Thad published nothing but a preliminary notice of the grating 
at that time, Mr. Glazebrook had not then seen my paper on 
the subject, of which I gave an abstract at the London Phys- 
ical Society in November last. In this paper I arrive at the 
conclusion that there is practically no aberration and that in 
this respect there is nothing further to be desired. 
he reason of this discrepancy is not far toseek. Mr. Glaze- 
brook assumes that the spaces are equal on the are of the circle. 
But I do not rule them in this manner; but the equal spaces 
are equal along the chord of the are. Again, the surface is not 
cylindrical, but spherical. 
These two errors entirely destroy the value of the paper as 
far as my gratings are concerned, for it only applies to a theo- 
retical grating, ruled in an entirely different manner from m 
own, and on a different form of surface. 
end of the paper for constructing aplanatic Hela op any 
the con- 
ART. XXV.—On the Stibnite from Japan; by EDWARD 
S. Dana. 
Tur Yale Museum has recently come into possession of a 
series of specimens of crystallized stibnite from Japan which 
are of so remarkable a character as to deserve a detailed de- 
here described formed part of a lot received by Mr. L. Stadt- 
miiller of this city from a correspondent residing.in Japan- 
er specimens, also of unusual excellence, were earlier re~ 
