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THE POPULAR SCIENCE MONTHLY. 



one employed at the Shattuck Observatory of Dartmouth College, and 

 most of our American observatories are supplied with instruments 

 similarly arranged. The light passes from the collimator c, through 

 the train of prisms p, near their bases, and, by two reflections in a rec- 

 tangular prism, r, is transferred to the upper story, so to speak, of 

 the prism-train, and made to return to the telescope t, finally reach- 

 ing the eye at e. It thus twice traverses a train of six prisms, and 

 the dispersive power of the instrument is twelve times as great as it 

 would be with only one prism. The diameter of the collimator is a 

 little less than an inch, and its length 10 inches. The whole instru- 

 ment, powerful as it is, only weighs about 14 pounds, and occupies a 

 space of about 15 in. x 6 in. x 5 in. It is also automatic, i. e., the tan- 

 gent screw m keeps the train of prisms adjusted to their position of 

 minimum deviation by the same movement which brings the different 

 portions of the spectrum to the centre of the field of view. 



The spectroscope is attached to the equatorial telescope, to which 

 it belongs, by means of the clamping rings a, a. These slide upon a 

 stout metal ro'd, firmly fastened to the telescope in such a way that 

 the slit s, of the instrument, can be placed exactly at the focus of the 

 object-glass, where the image of the sun is formed. 1 



The telescope is directed so that the solar image shall fall with 

 that portion of its limb which is to be examined just tangent to the 

 opened slit, as in Fig. 3, which represents the slit-plate of the spec- 

 troscope of its actual size, with the image of the sun in position for 

 observation just touching the rectangular opening formed on widening 

 the slit by its adjusting screw. 



Opened Slit of the Spectkoscope. 



If, now, a prominence exists at this part of the sun's limb (as would 

 probably be the case, considering the proximity of the spot shown in 

 Fig. 3), and if the spectroscope itself is so adjusted that the C line 

 falls in the centre of the field of view, then, on looking into the eye-piece, 

 one will see something much like Fig. 4. The red portion of the 



1 The writer has recently found that a so-called diffraction-grating may take the place 

 of the train of prisms in spectroscopes designed for simply viewing the prominences. 

 With a grating ruled upon speculum metal, having 6,480 lines to the inch (for which he 

 is indebted to the skill and kindness of Mr. Rutherfurd), he is able to observe the forms 

 and motions of these objects nearly as well as with the spectroscope described in the text. 



