L. M. Rutherfurd’s Spectroscope. 129 
Art. XVI.—On the construction of the Spectroscope ; by Lewis 
M. RUTHERFURD. 
I KNow of no good substitute for bisulphid of carbon as the 
dispersive agent in the spectroscope. Flint glass, besides being 
but half the dispersive power, and the specimens of the denser 
glass which I have seen tarnish so rapidiy, and have so high an 
index of refraction as to be practically useless, Having devoted 
much time to the construction and management of bisulphid of 
carbon prisms, it is quite possible that the results of my experi- 
ence may be useful to those who may wish to fit up a spectro- 
Scope with such prisms, and perhaps I shall best attain the object 
y describing my own instrument. 
The two principal telescopes are provided with objectives of 
16 inches aperture and 19 inches focal length. The slit or col- 
lecting telescope has but one motion about a vertical axis at the 
side of the platform and just in front of the objective, enabling 
it to command all parts of the platform. The observing tele: 
Scope has two motions, one about the central axis of the instru- 
ment, and the other about a second vertical axis, which by means 
of'aslide, capable of being clamped, can be placed under the Jast 
surface of any prism on the platform; thus commanding by 
one motion the whole spectrum. : 
Before the slit is a prism for the comparison of different spec- 
tra, and the observing telescope is provided with eye-pieces of 
various powers. The first circuit consists of six prisms which 
are of brass faced with plates of glass, cemented with glue and 
molasses, These are each of about the angle of 60° and present 
&n aperture of 2-9x1-8inches. The faces to receive the glass 
are carefully ground to a flat surface and the glass quite thick 
and free from veins has been selected with reference to the flat- 
ince however it is scarcely possible to find glass with paral- 
: I surfaces, care has been taken so to place the glass that the 
Inclination of its faces is perpendicular to the axis of the prism. 
to be glazed was u t and ina horizontal position: the 
blass, having been ones, wate the manner of a plate for photo- — 
