camera may be connected by a light-proof bellows or other enclosure, so that the instrument may be used in an undarkened room. This bellows may be supported partly upon the blocks which carry the linkage. The only part of the construction that may seem to offer difficulty is in making the linkage true, but this should not prove a serious obstacle. All that is necessary is that the four arms of each parallelogram shall be of equal length and that there shall be no play at the joints, and it should easily be possible to do this with sufficient accuracy. It may be added that the linkage, though extremely convenient, is not essential. The grating may be turned by hand to the angle corresponding to the wave-length desired, and then moved along the track until the light is focused on the slit. The camera may then be rotated until the spectrum is in focus.11 In this way it may be possible to use gratings of much larger radius, and so avoid the errors produced in ruling on a surface of too great curvature. The great compactness of the mounting makes it available for use in astronomical spectroscopy, from which the concave grating is practically barred when the Rowland mounting is used. The instrument may be mounted upon a telescope in the prolongation of its axis so that the slit lies in the focal plane of the objective. In the case of a star image the slit could be dispensed with, and the astigmatism of the grating would produce a spectrum of finite width. A more rigid and more convenient arrangement would be to mount the guides for the grating upon the tube of the telescope on the side opposite to the declination axis. The light could be brought to a focus by the objective at the side of the field nearest the slit and thrown upon the slit by totally reflecting prisms. No harm would be done by any possible astigmatism which would merely be added to that of the grating, and slight irregularities in driving would be equally harmless. It remains to consider the character of the spectra produced. The chief advantages of Rowland's mounting are that the spectrum is normal and always of the same scale, and that the plate is per"This is in fact the arrangement described by Eagle (1. c.). pendicular to the direction of the light. In the autocollimating mounting none of these conditions are fulfilled. These disadvantages are shared by all prism spectrographs. They are however less with the grating, and, a matter of great importance, the amount of the variations may be readily calculated and allowed for. The deviation of the spectrum from a normal one might seem to be a serious objection, but as a matter of fact the deviation is much less than might be thought. Thus on a plate of the usual size, the maximum deviation from a normal scale is about one Ångstrom, and when using a comparison spectrum the maximum difference between the corrections for two lines say ten Ångstroms apart would be about one twentieth of an Angstrom. Moreover the deviation from the normal scale may be accurately allowed for, being of the form s1 and s2 being the distance of the line in question from the two lines selected to establish the scale, and A, the wave-length which is returned through the slit. This correction is the same as that necessary when using a plane grating in the autocollimating position. The varying amount of dispersion is an objection in some classes of work, especially where plates are taken in different regions of the spectrum. The scale varies as sec p, and therefore is somewhat larger than that of the same order in the normal mounting, especially when the angle is large. Moreover, in a given direction the order of the spectrum is doubled and therefore the dispersion is more than twice as great. This is an important property of the autocollimating mounting, since twice as many orders are available for observation. Thus with a 15,000 grating, four complete spectra may be observed instead of only two. The inclination of the photographic plate will sometimes be a more serious objection. Great care should be taken in its register. This objection as somewhat weakened by the increase of the dispersion with the inclination, so that an error due to imperfect register is proportional to sin instead of tan 4. The error is greatly reduced when using comparison spectra on the same plate. Finally as to the definition. Kayser12 gives for the greatest permissible length of a grating used in the customary position-i. e̟., the length when the difference in phase between the extreme rays amounts to the expression π and the same formula holds for the autocollimating position. Comparing the same order of spectra we see that as is less in the second case (and therefore cot greater) the limiting length of grating would be greater; so that a given grating will be farther from this limiting value and hence will have better definition. The grating is in fact in the position of minimum deviation, and the aberration is equally divided between the incident and diffracted beams and is therefore a minimum. To sum up the mounting here described differs from the usual mounting for a concave grating by employing the principle of autocollimation. It possesses the advantages of the Rowland mounting of having all spectra automatically in focus, but differs from it in having greater compactness, convenience and rigidity. The adjustments are easier and the necessity of a dark room is avoided. The definition in the same order is somewhat greater, and twice as many orders may be observed. The deviations of the spectrum from the normal type are small and may be accurately allowed for. The instrument may be readily adapted to work in astronomical spectrography. RANDAL MORGAN LABORATORY OF PHYSICS, UNIVERSITY OF PENNSYLVANIA. 12 H. Kayser, "Handbuch der Spectroscopie," I., 458. THE OBJECTIVE PRISM. BY EDWARD C. PICKERING. Three methods may be employed for studying the spectra of the stars. First, the slit spectroscope. This is the method most widely used. The light of the star is concentrated on the slit of the spectroscope, and the linear spectrum widened, if necessary, by a cylindrical lens, or by moving the image of the star. Secondly, by the diffraction grating. As in the first method, the image of the star is concentrated on the slit. But little use has been made of this, and other diffraction methods in studying stellar spectra, owing to the great loss of light. Third, the objective prism. A prism of small angle is placed over the objective of the telescope, and the image of every star in the field is thus spread out into a linear spectrum. Any desired width may be given by allowing the star to traverse the plate slowly, parallel to the edges of the prism. This method cannot well be applied to reflectors, or to other telescopes of large size, owing to the size of prism required. Another objection, in the case of reflectors, is that the prism must be placed so far from the mirror that the definition is injured. These difficulties may be remedied by the focal plane spectroscope, in which the cone of rays from the star is rendered parallel by a concave lens, then passed through a mirror, and brought to a focus by a convex lens. All the light falling on a large mirror may thus be concentrated into a small space, so that the spectrum of a very faint star may be photographed. But little use has been made of this method, although it appears to have great possibilities. The principal advantages of the objective prism are the small loss of light, and the large number of stars which may be photographed simultaneously. Also, that it is not necessary to follow, as when photographing star charts. The best authorities claim that of the entire light entering the telescope, less than one per cent. reaches the photographic plate, when a slit spectroscope is used. The proportion of light transmitted by the objective prism must be at least fifty times as great. In fact, the principal loss of light is from the absorption of the objective. Consequently, far fainter stars can be photographed with an objective prism than with a slit spectroscope, the difference amounting to several magnitudes. Another great advantage of the objective prism is that the spectra of all the stars in the field of the telescope can be photographed simultaneously, while with a slit spectroscope only one star can be taken at a time. With the Harvard 8-inch doublet as many as three or four hundred spectra are often photographed on a plate, including all stars of the ninth magnitude and brighter, in a region ten degrees square. A comparison spectrum cannot be used with an objective prism, and it is accordingly difficult to obtain absolute wave-lengths, which are needed to determine the motion of stars in the line of sight. This constitutes the principal objection to the objective prism. Various plans have been proposed to remedy this difficulty, and how far they are successful will be described by another speaker. This does not affect the ordinary measures of wavelengths, as hydrogen lines are present in the spectra of nearly all the stars, and since these lines are affected by the motion, other lines. can be referred to them. The first photograph of the lines in the spectra of the stars was taken by Dr. Henry Draper of New York. In 1886, Mrs. Draper established, at the Harvard College Observatory, the Henry Draper Memorial, to prosecute the study of stellar spectra. The objective prism has been used almost exclusively in this work. Two photographic doublets of eight inches aperture have been mounted, one at Cambridge, the other at Arequipa, Peru. With these the entire sky has been covered many times. On one plate more than a thousand spectra were classified. The late Williamina P. Fleming, Curator of astronomical photographs, from an examination of these plates, discovered several thousand objects having peculiar spectra. In fact, probably few bright objects of this class escaped |