558 RICHARDS— AUTOCOLLIMATING MOUNTING [April 20. 



These adjustments however may be automatically made in the 

 following manner. As in Rowland's mounting, let the slit be fixed 

 at S (Fig. 3) and the grating be capable of sliding along the line 

 GS and also of rotating about a vertical axis passing through its 

 center. Now if two equal horizontal arms of length p/2 be pivoted 

 at G and 5" respectively and hinged together at their other ends at 

 O, and the arm OG be attached to the grating holder so as to be 

 parallel to the grating-normal, the grating will keep the proper 

 inclination as it slides along GS, for G and 5" are constrained to 

 remain on the Rowland circle. This is in fact exactly equivalent 

 to one half of a Rowland mounting. Moreover, if the camera is 

 mounted to rotate about a vertical axis through S, and the arm OS 

 is similarly attached normally to the photographic plate, the plate, 

 if bent as usual into the arc of the proper circle, will continue to fit 

 this circle throughout its motion and the spectrum will be in focus 

 on all parts. 



In practice the arms OG and OS would be excessively long and 

 inconvenient, and would tend to bend the vertical axes at G and 5". 

 They could of course be balanced by a pair of similar arms on the 

 other side, but the apparatus would then be still more cumbersome. 

 The same effect may however be attained by a series of links of the 

 " lazy-tongs " pattern, the total length when open being equal to the 

 radius of the grating (Fig. 4). The first and last link on one side 

 will correspond in direction to the arms OG and OS, and these are 

 fixed normal to the grating and plate respectively. It is obvious 

 that as either side may be used, all of the grating spectra become 

 available. 



A wooden model of the apparatus has been constructed for use 

 with a six-foot grating, which exhibits the proposed arrangement 

 and which in spite of its crudity renders excellent service. A more 

 efficient mount is in process of construction. Sliding along a hori- 

 zontal track is a block carrying a vertical pin on which as an axis 

 turn the ends of the link motion and the platform for the grating. 

 This latter may be clamped in any position to either of the adjacent 

 link' bars. At one end of the track is a fixed block with a similar 

 vertical axis. This axis carries the other end of the linkage, a 



