356 Prof. O. N. Rood on Prisms of Flint Glass, etc. 
Art. XXXV.—On the use of Prisms of Flint Glass and Bisulphid 
of carbon for Spectral Analysis; by Prof. O. N. Roop. 
In a letter to Prof. B. Silliman, Jr., which was published by 
him in the September No., 1862, of this Journal, I deseribed a 
new form of bisulphid of carbon prism, provided with compound 
faces, which corrected the distertion usually attendant on such 
prisms. I ventured at that time to suggest that large prisms of 
this kind approached a degree of optical perfection not attainable 
by the best flint glass prisms yet produced. Some late experi- 
ments of Sigmund Merz,’ one of the successors of Fraunhofer, 
furnish a confirmation of my opinion, which I certainly did not 
expect to receive from that particular quarter. In my letter] 
mentioned the discovery of two new lines in the interior of the 
line D, which made in all three fine lines that were thus enclosed, 
one having previously been laid down by Kirchhoff. To effect 
this, three bisulphid of carbon prisms of 60°, with a flint glass 
prism of 45° were employed; the sum of the refracting angles 
was then 225°. Now Merz states that by the use of a number 
of glass prisms, the sum of their refracting angles being 270°, OF 
45° greater than that employed by me, he discovered a secon 
line in the interior of D, but nothing more; the third line 1t ap- 
pears was invisible. This second line observed by him I may 
remark, in my spectroscope was apparently as strong as that late 
down by Kirchhoff, so that it was a matter of some wonder that 
. it had escaped resolution in his hands. a 
Merz then employed eleven glass prisms, the sum of ree 
refracting angles being 480°; with these he discovered the 7 
line I had previously seen, along with two additional quite Ane 
lines. He therefore describes the line D to consist of: two q! ‘ie 
broad lines, (those commonly known,) two of less breadth, 4 
three fine lines. : Re 
_ When we consider that this optician had at his command tht : 
best flint glass prisms in the world, and observing telescopes - 
have hardly ever been surpassed, the argument to be pede = 
favor of bisulphid of carbon prisms properly corrected, is I think | 
a strong one; particularly when I mention that the teles - 
used by me were the common cheap French article, various’? 
amended to secure an approximation to achromatism. | an : 
__ Farther, according to the observations of Merz, a single a 
_ prism (48 lines) used with a large condensing telescope 
: ines in diameter), shows D resolved into five lines, demonstrating cme 
thus the value of size in the apparatus; this seems again ™ “a 
an excellent reason for the use of bisulphid of carbon OP©™ 
ground of its far greater cheapness, 
© Pence Dale, R. I., March 10th, 1863, — 
jer das Farbe lesnstvan on Bi 1 Merz i - Miinchen. Pog A nnalen, 
-und 6 d. polytechn. Vereins fir d. Kénigr. Bayern, Oct. See 
