| 
: 
anc ght angles. The ve 
_ being placed at H, the eye is brought opposi 
O. N. Rood on a Photometer. 61 
reflected light, and the sudden, sharp transition from the thin 
opaque edges of the film to the clear glass, the screenis, I sup- 
pose, better adapted for use than any which has yet been pro- 
posed. The collodion side of the glass plate is turned towards L. 
2d. Next in order comes a piece of blackened card board of 
the same size with plate No. 1: this is provided with an aper- 
ture corresponding with, and somewhat larger than, the exposed 
portion of the collodion plate. 
3d. Then follow two thin finely ground glass plates of the 
same size with No. 1, the ground surfaces facing towards L. A 
single plate of ground glass can be used, but with two plates the 
illumination of the square spot is much more uniform and even; 
in other words, by the use of two plates all idea of the ground 
glass texture is removed, and nothing is seen but a square patch 
1 
Two small lamps or standard candles are placed about as in- 
dicated at L. The construction of the lamps is the same with 
those used by Potter, (Physical Optics, page 112,) in his photo- 
metric experiments, consisting, viz: of shallow cups filled with 
oil, which support little metallic bridges, formed of thin plates 
of metal perforated with four holes, through two of which small 
wicks are drawn, the others supplying air. When properly ar- 
ranged these lamps will give a pretty uniform light for thirty 
minutes together; but though by no means furnishing a truly 
constant illumination, yet the variation is almost always gradual 
and steady, a point of great importance, for this being the case, 
it is easy to make the observations in such a manner as nearly 
to exclude errors from this source, as will presently be shown.” 
The movable lamp is provided with a vernier, and a scale 
divided into inches and tenths extends from that aber glass 
surface next to the collodion plate along the entire length of the 
At L is a shade to protect the eyes while observing ; 
this has an aperture one inch square placed in the axis of the 
instrument. The photometer, when arranged as above described, 
is peculiarly adapted to measure the amount of light transmitted 
by plates of colorless or colored substances, as well as the amount 
of light reflected from polished or unpolished plane surfaces at 
atick For these uses it was expressl 
_ Mode of using the instrument—The three lamps, after being 
lighted, are allowed to burn fortwenty orthirty . | 
minutes, till their light has become steady, then 
to coincide with the vernier by using the arrange- Es 
Tm 
ier | 
te the dotted line 
