132 Contributions from the Sheffield Laboratory. 
system of prisms without the button, as seen from above, 1 
shew the manner in which this adjustment is accomplished. | 
the glass plate which forms the platform of the instrument, 
in the center of the system, is cemented a brass plate, in ac 
of which revolves without shake a pinion provided at t 
with a milled head, as 2. 
seen in figure 2. The a 
prisms are all hinged to- 2. 
getuer at the corners: 
and from the back of 
each projects at right 
angles a brass bar pro- 
vided with a slot, which 
is provided with teeth which gear into the pinion, so that 
turning the milled head this prism is forced to approach de. 
part from the center: but, from the construction, this cannot take 
place without imparting a similar motion to each of the 
prisms, and thus, at will, their backs are made tangents to a 
ger or smaller circle, which is the adjustment sought. 7 
This mechanism is capable of adjusting six, or any smallet 
number, of eqni-angled prisms. The outer spiral, when moe 
than six are used, must be adjusted by hand. 
New York, Dec. 10, 1864, 
Art. XVII.— Contributions from the Sheffield Laboratory of Yal 
College. No. VILL—On crystallized Diopside as a furnace 
duct; by Georce J. Brusu. 
while others were grayish white and transculent; some 
larger ones were over half an inch in length by ones! 
of an inch in diameter. Mr. John M. Blake has determine? 
