J. W. Draper—New form of Spectrometer, 83 
glass ground on both faces, an extinguishing beam of sunlight 
assed. This ground glass served to disseminate the extin- 
guishing light uniformly over the spectrum. I could regulate 
the power of this light by varying the ae of the aperture 
through which it came by means of a sli 
It is needless to give details of the ae obtained by this 
instrument. viet nee identical with those described in the 
maces paragra 
It might be sup past that the irrationality of ne of dif- 
ferent prisms Suppo d influence the results perce Accord- 
ingly, I tried prisms of different kinds of glass oa other trans- 
parent substances, but could not find that this was the case, In 
all, the extinction began in the violet and ended in the red. 
or did there seem to be any difference when the effect ps 
viewed by different eyes. To persons, irrespective of age o 
the condition of their sight, the extinction took place: in the 
same manner. I had not an opportunity of examination in a 
case of color-blindness, 
2d. Case of the Grating or Diffraction Spectrum.—lf the cause 
of the increasing intensity of light in the prismatic spectrum 
from the more to the less refrangible Neca be i 
exercised by the prism on the colore reasing as the 
refrangibility is less, we ought not to find sod pene peculiarity 
in the diffraction spectrum. In this the colored re 
ces 
arranged uniformly and equably in the order of their wave- 
at the same moment. 
Having modified the common spectroscope by taking away 
its dark box so that the slit tube and telescope tube could be set 
in any required angular position, I put in the place of its prism 
a glass grating inclined at forty-five degrees to rays coming in 
through the slit. The ruled side of the grating was present 
the slit. Now when the extinguishing flame was properly placed 
before the ground glass, the plane side of the grating Tr its 
light down the telescope ih al In this, as in the former case, the 
spectrum was seen in the midst of a field of light, thei Ststalty of 
which could be varied by varying the distance of the extinguish - 
ing flame, or by varying the opening of its stop-cock. This 
light needs no reénforcement by increasing the reflecting power 
of the back face of the grating, these spectra being much more 
feeble than that given by a prism, and the Peeasited light 
being quite able to extinguish them. 
As the glass grating I was using gave its two series of 
spectra of unequal a eee I selected the most brilliant, 
and in it used the spectrum of the first order. I saw, not with- 
out pleasure, that as the force of the extinguishing illumin- 
ation increased, all the colored spaces yielded apparently in an 
equal manner, ‘and disappeared at the same moment. Some- 
Am, Jour. Sc1.—Tuir — Vou. XVIII, No. 103.—JuLyY, 1879. : 
