J. W. Draper—New form of Spectrometer. 81 
Remove from the common three-tubed spectroscope its scale 
tube, and place against the aperture into which it was screwed 
a piece of glass, ground on both sides. In front of this arrange 
an ordinary gas light, attached to a flexible tube, so that its 
distance from the ground glass may be varied at pleasure. On 
looking through the telescope tube, the field of view will be 
uniformly illuminated, this being the use of the ground glass. 
The brilliancy of the field depends on the distance of the gas 
light, according to the ordinary photometric law. 
Ast. Case of the prismatic or dispersion spectrum.—If the extin- 
guishing light be for the moment put out, and 9 the proper 
lace before the slit tube the duminous flame of the Bunsen 
urner that accompanies the apparatus be arranged, on lookin 
through the telescope a spectrum of that luminous flame will 
| of course be seen. The slit itself should be very narrow, so 
. that the spectrum may not be too bright. 
Now let the extinguishing flame be placed before the ground 
glass, and a spectrum is seen in the midst of a field of light, the 
brilliancy of which can be varied at pleasure. If the extin- 
guishing flame be at a suitable distance, the whole —_ 
may be discerned. As that distance is shortened, first 
violet, and then the other more refrangible colors in their 
descending order disappear, and at length in the steadily in- 
creasing effulgence the red alone remains. The yellow never 
stands out conspicuously as might have been ex 
This is scarcely consistent with the assertion that the yellow 
is the brightest of the rays. The red is plainly perceptible long 
after the yellow has gone. There is a greenish tint emitted by 
gas-light that disappears a little previously to the extinction of 
the red. 
From these observations I think that the luminous —— 
y 
i 
: 
; 
a 
: 
ing 
that, ecient considered, the intrinsic bia of re Ig “ is 
the same for all. In this we must s bear in mind the 
physiological peculiarities of the eye. 
The foregoing statement is apy 2 hil explicit to , 
enable any one to verify the facts. y, however, mention, 
some improvements in the apparatus, Bah L experience has led 
me to 
The intensity of the extinguishing ee may be pag 
to obliterate the spectrum, even though the slit be cl sae Se: 
rowed. How then may the intensity of the spectrum pe imin- 
ished, and that of the es light be simultaneously in- 
creased? I accomplished this by depositing on that face of 
the prism which acts as a reflector an excessively thin film of 
silver. This, though it was transparent to the transmitted 
