560 RICHARDS— AUTOCOLLIMATING MOUNTING [April 20, 



The grating and slit are provided with the usual adjustments- 

 and also may be shifted on their platforms until their centers fall 

 exactly in the axes of rotation. The camera is mounted on its 

 platform on either side of the slit or preferably just above it, and 

 may be adjusted so as to bring the plate into coincidence with the 

 focal circle. If the latter position is used the grating must be 

 raised until its center is in a horizontal plane midway between the 

 slit and camera. For visual observations the plate holder may be 

 replaced by an eyepiece which may be fixed if mounted above the 

 slit, or if placed at one side is kept directed toward the grating by 

 a simple device. The slit is provided with a reflecting prism and is 

 made double so that light may be sent through one part and returned 

 through the other. It is thus possible to obtain a monochromatic 

 beam of high purity which will be sent always in the same direction. 

 The apparatus is thus available for a threefold use: as a spectro- 

 graph, as an observing spectroscope, and as a monochromator ; and 

 it may be arranged so that no alteration need be made in passing 

 from one to another of these forms. 



The diagonal of any parallelogram of the linkage perpendicular 

 to the track is proportional to sin <f), and therefore to the wave- 

 length. A scale of equal parts placed across any part of the link- 

 age perpendicular to the track, as at Sc (Fig. 4), will therefore give 

 an approximate measure of the wave-length. Moreover by properly 

 selecting the points on the bars across which the scale is placed, any 

 scale may be adjusted to read wave-lengths directly. A more open 

 scale may be placed on the track, but this will not be one of equal 

 parts. 



As everything is supported on 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 



