NATURE 
[Dec. 8, 1870 


SPECTROSCOPIC NOTES“ 
A NEW FORM OF SPECTROSCOPE 
HE instrument, a’description of which follows, was designed 
for attachment to the equatorial of 6:4in.!aperture and oft, focal 
length, belonging to the observatory of Dartmouth College. It 
is especially intended for observations upon the solar spots and 
protuberances, and accordingly the principal object kept in view 
has been to combine a very high degree of power with compact- 
ness, lightness, facility of manipulation, and firmness of construc- 
tion. Having the dispersive power of 13 prisms of heavy flint, 
each with an angle of 55°, it yet weighs less than 15lb., and 
measures over all 15in. in length, 8in. in breadth, and 4}in. in 
height. It was make by Alvan Clark and Sons. 
The accompanying plate (Fig. 1), taken from a photograph, 
gives a correct idea of its appearance and general arrangement. 
The collimator and observing telescope have each an aperture of 
{ths of an inch, and a focal length of 7in., which might 
advantageously have been increased to 12in. were it not for the 
necessity of compactness. 
The light from the slit, after passing the collimator, is trans- 



mitted through the lower portion of a train of six prisms of heavy 
flint glass each 24in. high, and having, as stated above, a 
refracting angle of 55°. A seventh half-prism follows, and to 
the back of this is cemented a right-angled prism, by which, 
after two total reflections, the light is sent back through the upper 
part of the same train of prisms, until it reaches the observing- 
telescope. This is placed directly above the collimator, and 
firmly attached to it. Finally, a diagonal eye-piece brings the 
rays to the eye in a convenient position for observation. 
The instrument has thus the dispersive power of thirteen 
prisms, and even with the low magnifying power of only five on 
the observing telescope, shows perfectly the lines of aqueous 
vapour, which make their appearance between the sodium lines 
when the sun is near the horizon. Of course, everything shown 
on the maps of Kirchhoff and Angstrém is readily seen with it, 
and many lines besides. 
Its definition is very beautiful, and the only optical fault of the 
instrument seems to be a curvature of the lines, resulting from 
the shortness of the collimator. - 
After planning the instrument, I learned that the same idea of 
sending the light twice through the prisms by a right-angled 
fr 
= 
E 
ria. | 
FIG. I1.—A NEW FORM OF SPECTROSCOPE 
prism at the end of the train had also occurred to Mr. Lockyer 
and others; but I do not know that it has yet been put in 
practice elsewhere. + 
The prisms, for protection and convenience of handling, are 
set in frames of blackened brass. They are arranged around the 
circumference of a hollow cylinder of elastic gun metal, 33in. in 
diameter, with stout flanges above and below, between which 
they are clamped by little thumb-screws, so that they can be 
readily removed or transposed : it requires less than a minute to 
put the last prism with its reflector in place of any other of the 
train, thus reducing the dispersive power to any extent desired. 
No particular care is required in placing the prisms, as a couple 
of narrow flanges were cast upon the cylinder near the top and 
bottem, and afterwards planed off to form true bearings for the 
backs of the prisms. They are thus always correctly set by 
being simply slid home before tightening the clamping screws. 
The lower flange of the cylinder is attached to the base-plate 
by a screw directly under the middle of the front face of the first 
prism. Around this point as a centre the whole system of prisms 
* From the Journal of the Franklin Institute. 
+ Aninstrument exactly similar in all essentials to the one here described 
has been used by Mr. Lockyer for more than a year past. 


is movable by means of a double-threaded tangent-screw, which 
brings the different portions of the spectrum into the field of 
view. The adjustment of the prisms to their angle of minimum 
deviation is effected by a method devised by Mr. George Clark, 
which is exceedingly simple, and, if not theoretically exact, 
answers every practical purpose. The flanges between which the 
prisms are clamped, are sawed through between the prisms, and 
a portion of the cylinder, flanges and all, equal to an arc of about 
30°, is cut out between the first prism and the last. On closing 
up or spreading open this gap by means of a suitable tangent- 
screw, the circumference of the circle around which the prisms 
stand is correspondingly enlarged or diminished. Probably, 
when the ends of this opening are drawn very near together, or 
spread very far apart, the cylinder is somewhat distorted, and a 
corresponding mal-adjustment of the prisms results ; but if so the 
effect is very slight. 
The instrument gives a perfect view of every part of the spec- 
trum from below A to H: above 4, however, when all seven - 
prisms are used, there is a loss of light occasioned by a partial 
obstruction of the apertures of the collimator and telescope by 
the corner of the reflecting prism. 
