128 



SCIENCE 



[N. S. Vol. XXXVI. No. 917 



aboiit a vertical axis through its center. At 

 one end of the track is a fixed vertical axis, 

 carrying a platform S for the slit and another 

 CC for the camera, both capable of rotation. 

 The two axes are joined by a system of link 

 bars of the " lazy-tongs " type, the total length 

 of which, when fully extended, is equal to p, 

 the radius of the grating. It is obvious that 

 when the inclination of the bars to the track 

 is ^, the distance GS will be p cos cj). If then 

 the first and last bars of the linkage on one 

 side are connected to the camera and grating 

 so as to be parallel respectively to their nor- 

 mals, the inclination of both camera and 

 grating will be correct however the distance 

 GS is altered. Since either side may be used, 

 all the grating spectra become available. 



The linkage is supported at its intersec- 

 tions by blocks which slide along the track. 

 The grating slider is moved by a rod or screw 

 running to the end of the track near the slit, 

 the only function of the linkage being to pro- 

 duce the necessary rotation. There is there- 

 fore no great stress on the axes tending to 

 bend them. The grating and slit are pro- 

 vided with the usual adjustments and also may 

 be shifted on their platforms until their cen- 

 ters fall exactly in the axes of rotation. The 

 camera is mounted on its platform on either 

 side of the slit at C or C" or preferably just 

 above it, and may be adjusted so as to bring 

 the plate into coincidence with the focal circle. 

 For visual observation the plateholder may 

 be replaced by an eyepiece. The slit is made 

 double so that light may be sent through one 

 part and returned through the other. The 

 apparatus is thus available for a threefold 

 use, as a spectrograph, as an observing spec- 

 troscope, and as a monochromator. 



The diagonal of any parallelogram of the 

 linkage perpendicular to the track is propor- 

 tional to sin <^; and therefore to the wave- 

 length. A scale of equal parts placed across 

 any part of the linkage perpendicular to the 

 track, as Sc (Fig. 2), will, therefore, give an 

 approximate measure of the wave-length. A 

 more open scale may be placed on the track, 

 but this will not be one of equal parts. 



As everything is supported upon one track. 



the apparatus may be made quite rigid, and 

 at the same time, with the smaller gratings at 

 least, portable. It will take up much less 

 space than the other mountings. It is also 

 more convenient, as everything is in reach at 

 the same time from the end of the track — 

 source, slit, camera and handle for controlling 

 the position of the grating. There is but one 

 track to make true, and the other adjustments 

 are no more difficult, and in some cases much 

 easier, than in the Rowland mounting. A 

 very desirable feature is that the slit, grating 

 and 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 great compactness of the mounting 

 makes it available for use in astronomical 

 spectroscopy. 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. 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. 



The definition of the spectrum is somewhat 

 greater than with the usual mounting, when 

 spectra of the same order are compared. 

 Moreover twice as many orders on each side 

 may be observed. The principal disadvan- 

 tages are that the scale of the spectra is not 

 constant, so that the spectra are not normal. 

 The deviations, however, are quite small and 

 may be accurately allowed for. Another ob- 

 jection is the inclination of the plate, which 

 requires special care in its register. These 

 objections, which are shared by prism spectro- 

 graphs, are much less serious when compari- 

 son spectra are used on the same plate. 



Horace Clark Eichards 



Eandal Morgan Laboratory of Physics, 

 University of Pennsylvania 



