The combined solar spectroscope and spectroheliograph at present used with the Yerkes telescope is the instrument designed by the writer in l889, and used in his work at the Harvard and Kenwood Observatories. It has recently been remodeled in the instrument shop of the Yerkes Observatory,the lever system and moving slits formerly used being replaced by a moving collimator slit and plate of the type designed by Professor Wadsworth. On account of the comparatively small angular aperture of the collimator and camera objectives, this spectroheliograph is not well adapted for use with the forty-inch telescope. The solar image at the focus of the telescope is nearly seven inches in diameter, and of this a zone only two inches wide and three inches long can be photographed in a single operation. It will therefore be necessary to obtain a larger spectroheliograph as soon as possible.

As the 40-inch Yerkes telescope is to be used during the greater part of the time for astro-physical investigations, three spectroscopic attachments are to be provided for it. Of these the star spectroscope has been completed by Mr. Brashear, and the construction of the solar spectroscope and spectroheliograph will shortly be undertaken. The solar spectroscope will differ from existing instruments of the same class mainly in its increased size and in certain novelties of construction. In the nature of the case the general design of such a spectroscope must follow certain well-known conditions, which do not admit of wide deviation from existing types. But the designer of a spectroheliograph has more freedom of choice. The instrument is susceptible of decided variation in form, as I have already had occasion to point out. The various types of this instrument described in my paper entitled "The Spectroheliograph" (see Astronomy and Astro-Physics, March, l893 , p.241)1 have each certain advantages and disadvantages, and these must be carefully weighed against one another in designing a large and important instrument for a special purpose.

The 40-inch Yerkes telescope has a focal length of about 64 feet, and will consequently give a solar image nearly 6.5 inches in diameter. A moment's consideration of the facts presented in the paper just referred to will make clear the impracticability of constructing a fixed spectroheliograph with moving slits large enough to allow the whole image to be photographed on a single plate without loss of light near the limb. Suffice it to say that the telescopes of such a spectroheliograph would have an aperture of about 9 inches. In spite of the great weight of such an instrument this large aperture would not of itself be an insuperable objection, were it not for the fact that the ruled surface of the grating (or the edge of base of the prisms, in case they were chosen) would be between l2 and l8 inches long. Unfortunately we have yet to see gratings or prisms (of large angle)of these dimensions.

Nevertheless, it was deemed of the utmost importance that the entire extent of the sun-spot zones be secured on a single plate. That is, it is desirable to photograph an equatorial zone about 4 inches in width. Manifestly this could not be done with a fixed spectroheliograph (like that successfully used with a 2 inch solar image at the Kenwood Observatory) provided with slits moving in the focal planes of the telescopes. Hence the form of spectroheliograph described in the last paragraph on page 256 of the article already referred to has been adopted, substituting, however, one, two, or three simple prisms, with or without a plane mirror, for the grating and plane mirror there described. The arrangement of the prisms and mirror is such as to give a deviation of l80 degrees for the K line. The collimator and telescope are of equal aperture and focal length. The entire instrument is moved at right angles to the optical axis of the 40-inch telescope on wheels with ball-bearings, running on knife-edges. The frame which carries the knife-edges is attached to the equatorial by means of four steel tubes 4 inches in diameter. Provision is made for rotating the frame in position-angle, so that the motion of the spectroheliograph can be made parallel to the solar equator. The photographic plate-holder is fixed to the frame, and the second slit moves close to the surface of the stationary plate.

The most important advantages of this form of spectroheliograph is the large field photographed. The length of the field is evidently determined by the length of the knife-edges on which the intrument runs. This may conveniently be as much as ten inches, or even more. The width of the field is determined by the length of the slits, the height of the prisms, and the aperture of the collimator and telescope. For with the slit fixed in the axis of the collimator the width of the illuminated portion of the collimator objective remains--with a given focal length--constant. It is always concentric with the objective. The base of the prism is thus comparatively small. For instance, if the ratio of the aperture to focal length in the equatorial is l-l8, and the collimator is 36 inches long, the length of one side of a 60 degree prism would be about four inches. Prisms of this size may readily be obtained, and as an increase in the width of field means simply an increase in the height of the prism, and not in the size of base, a very large field can thus be photographed. In order to photograph in a single exposure, the 6.9 inch image given by the Yerkes telescope a collimator and telescope of 8.5 inches aperture and 36 inches focal length, with 60 degree prisms 4 inches on an edge and 5 inches high, would suffice. As such an optical combination would, to say the least, be extremely difficult to realize, it is probable that we must be content with photographs showing a zone not wider than four inches.

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