S 
_ Kirchhoff’s central line are not difficult, being readily : ‘ 
seen with three prisms of 60°, of bisulphid and one of 45°, of glass (it: 
being possi of the interference 
408 Scientific Correspond 
to fine performance are two-fold. 1st. I find no specimen of bisulphid 
of carbon homogeneous in density : upon shaking or disturbing the posi- 
tion of the prism a violent agitation of the image occurs, and in exam- 
ining it without the eye-piece, after the mode adopted in detecting veins 
in an object-glass, the whole interior of the prism is seen full of waves 
and striz, presenting the appearance of alcohol and water not yet thor- 
oughly mixed; this trouble is cured by time, from a quarter to a half 
hour being a sufficient rest. 2d. The brass frame is so much more 
oval opening, from which Mr. Clark has prepared CS, prisms, hol : 
nearly a pint of liquid, and exposing faces of about five inches — 
by three high; two of these proje¢t a spectrum from Deleuil’s eee 
which the 10 
p. 414 of this 
S., JR. a 
4. Analysis of the Sodium line D.—As I said above, my brass PUSH 
under favorable conditions perform admirably; with six of them lam 
confident that I haye seen the line D composed of . 
nine (see figure); this diagram is rude, not founded i \[3 a 
not 
upon measures, but merely a copy of a sketch made : | 
vhen I first saw the lines; the three on the right of © 
possible to use four of 60° on account tral on@ 
ie telescopes). Of the three in the left compartment, the cen Tight’ 
‘most difficult, and all require the best adjustment and 1g 
