60 O. N. Rood on a Photometer. 
Hydrastine = C,,H,,NO,, in 100 parts: 
Fo 
nd. Calculated. 
Carbon, roo 66°696  66°379 66°333 
Hydrogen, oot 6:010 5°700 6:030 
Nitrogen — 3°832 3°767 3°517 
Oxygen, = 23°462 ae 24°118 
The chloroplatinate of “hydrastine, ay 12» HCl+PrCl,, 
requires in 100 pa 32 p. c. platinum ; t pe 16°17 p.c¢. 
The chlorhydrate of hy rastine =C bats Oa ey HC),. COM 
| pale pga to its formula, 8°34 p. c. chlorhydrie acid, ‘while 
fo 
Chicago, te Yak 31, 1863. 
Art. [X.— Description of a Photometer ; by Prof. O. N. Roop. 
Dov, a short time ago, proposed a photometer, consisting 
essentially of a compound microscope provided with a micro- 
copic phonemes the latter being used as a “screen,” and illu- 
minated from both sides, in such a way that when the compen- 
sation Ssaek ror reached, the photograph was rendered invisible.’ 
n the construction of the instrument at present described, I 
have availed myself of this general mode of comparison, dis- 
carding however the microscope and photograph, and supplying 
their place in a more simple, and, for many purposes, in an at 
least equally efficient manner 
The instrument consists of a board from 50 to 100 inches in 
length and 12 inches wide; a slide at S eves a small lamp 
or standard candle; the slide can 
be moved towards or away from 
the screen P by a light rod R; 
the screen, reckoning from L, is 
with a collodion sensitized with iodid of potassium and of the 
variety called adherent; this is immersed in a solution of nitrate 
camera 
minutes, ihe devel i by sulphate of iron, : ahead and dried. 
By this operation a 
is produced in the substance of the collodion. The eollodion _ 
film is then removed from one spot $ an inch square; thiscan 
be done neatly and completely by the use of a needle, great care 
being taken to leave the edges of the little square ’ shar 
n. The observations are made by means of this spot, Beg: it : 
of course is to be located in the axis of the instrument. On 
greene. of the uniformity of the appearance of this plate by 
+ Pog. Annalen, Band exiv, page 145. : 
CEEECEEEEtT EL Ue 
eae SN et eee ete ee ss Sian 
SRI ty ee eA 
