Dee. 8, 1870] 
NATURE 
1 G2 ¢ 

Were it important to secure the perfect cylindricity of the 
prism-frame through the whole range of adjustment, it could be 
easily done by merely fastening at the back of each prism a radial 
bar acting upon a central pin, as in the arrangement first devised 
by Mr. Rutherfurd, and since adopted by Mr. Browning, in his 
automatic spectroscope. 
This plan of Mr. Clark’s, doing away with all joints and hinges, 
has the great advantage of perfect firmness and solidity in every 


aaa aa aera 
TELESCOPE 

position of the instrument, an advantage hardly to be oveirated 
in an astronomical spectroscope. 
Had it occurred to me in season I might have made the instru- 
ment still simpler, firmer, and perfectly automatic in its adjust- 
ment, by merely substituting for the first prism a half-prism,* 
like the last of the train, to which the right-angled reflecting 
prism is cemented. 

Placing the first half-prism with its front face perpendicular to 
the line of collimation, it would never need to be disturbed ; the 
flange of the cylindrical frame which carries the prisms would be 

firmly fastened to the bed-plate immediately beneath it, and the | 
pivot joint at this place with the corresponding tangent-screw 
would be dispensed with. The only adjustment required would 
be that produced by the screw which is now used to adjust for 
minimum deviation hy opening or closing the gap of the cylinder. 

Of course, this arrangement would reduce the dispersive power 
. of the train by the amount of one prism, a loss easily made up 
by adding a degree or two to their refracting angles. 
Vig. 2 exhibits the plan of the proposed arrangement, and re- 
quires no explanation, unless to remark that for the sake of dis- 
tinctness I have represented only two of the radial bars which 
may be used to render the adjustment accurate. 
* On returning from the Eclipse Expedition my instrument will be made 
automatic in aceordance with this plau. 
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| 
It might be better to place the face of the first prism not exactly 
normal to the line of collimation, in order to avoid repeated re- 
flections between it and the object-glass of the collimator, which 
would be likely to produce a troublesome ghost, or the same 
thing might be accomplished by simply cementing the object- 
glasses of both collimator and observing telescope directly upon 
the front of the prism ; this would make the instrument still more 
solid and compact. 
The eye-piece of the instrument has an apparatus attached, 
which, however, thanks to the high dispersive power, I find un- 
necessary. 

_It was early proposed by Janssen to use a vibrating or rotating 
slit in order to make visible the form of a solar prominence, but 
as Zollner has shown, the mere opening of the slit answers just as 
well, the light of the protuberance being diluted to precisely the 
same extent in either case. 
It occurred to me in connection with a suggestion of Professor 
Morton, that by interposing at the focus of the eye-piece a dia- 
phragm which should moye with the vibrating slit, the light of 
the neighbouring portions of the spectrum might be cut off and 
this dilution avoided. Mr. Clark has devised and constructed 
a very beautiful mechanical arrangement by which this simul- 
taneous and accordant motion of slit and diaphragm is effected 
by the rotation of the small fly-wheel shown in Fig. 1. 

But I find, that although seen in this way, the prominences 
| appear very bright; yet the working of the apparatus always 
causes a slight oscillation of the equatorial, which interferes with 
the definition of details, and I prefer to work with the shit simply 
opened. When the airis free from haze, the whole extent of a 
| prontinence 30,000 miles in height is readily examined through 
the C or F line, aad the most delicate details reveal themselves 
, with a beauty and clearness of definition which even yet always 
| surprises me, and speaks most emphatically for the exquisite 
workmanship of the 43 different surfaces by which the light is 
either refracted or reflected on its way from the slit of the col- 
limator to the eye. 
Fiz 



—$<s—___ 
a 
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2.39 
But, although I do not use the vibration of the slit and dia- 
phragm, I find the mobility of the slit so convenient as to be 
practically indispensable. In examining the spectrum of a group 
of sun-spots, for instance, it is very much easier to move the slit 
to the particular point we wish to observe, than to move the solar 


| image by the tangent screws of the equatorial. 
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| 
1 
Photographs of the Solar Protuberances 
The protuberances are so well seen through the F and 2796 
(near G) lines, that it is even possible to photograph them, 
though perhaps net so satisfactorily with so small a telescope as 
the one at my command, Some exp.riments I have recently 
