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 rectangular prism, r, is transferred to the upper story, so to speak, of the prism-train, and made to return to the telescope t, finally reaching 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 instrument, 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 tangent 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 rod, 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.
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 spectroscope 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.
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
- 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.