﻿Fixed- Arm Spectroscopes. 349 



while incidentally the more or less trouhlesome minimum- 

 deviation attachment is done away with. The last arrange- 

 ment, which is in one sense a direct-vision spectroscope with 

 but a single prism, fulfils the condition of parallelism be- 

 tween the refracted reflected ray and the incident ray, and 

 it is therefore the one which has been finally adopted for 

 the spectro-bolometer of the Observatory in place of the form 

 first used on that instrument*. 



A plan view of the mounting in this particular form of 

 instrument is shown in fig. 5, PI. IX., and a side view 

 (photograph) in PI. X. It will be seen that the prism and 

 mirror are mounted together in a single frame AA, provided 

 with three levelling-screws which rest, one in a conical hole, 

 the second in a slot, and the third on a plane ; so that the 

 whole frame may readily be removed from the spectrometer 

 table and then replaced in exactly the same position. The 

 triangular prism-table B is arranged to slide vertically in the 

 guides a b, and thus provide for prisms of different heights. 

 The table has a motion of adjustment regulated by the screw 

 c, about a line parallel to the mirror-face, for the purpose of 

 bringing the refracting edge of the prism parallel to that face, 

 and is provided with screws e, f, g, h, which serve to adjust 

 the prism laterally and in angle, so that in the first place the 

 plane bisecting the refracting angle may pass through the 

 axis of rotation, and, secondly, so that the prism-faces may 

 make equal angles with the faces of the mirror (the minimum- 

 deviation condition). It will be observed that when these 

 screws are once adjusted they serve as stops which will bring 

 any prism (of standard 60° angle) that may be used into the 

 correct position ; for, if smaller than the prism for which they 



* Since writing the above rny attention has been called to an article 

 in Zeit. fur Tnstrumentenkunde for November 1881, describing this 

 particular " direct-vision " arrangement of the prism and mirror. There 

 was, however, no indication of the general class of which this is but a 

 particular type, and hence, of course, no indication of the conditions 

 which it was necessary to fulfil to prevent a lateral shifting of the beam ; 

 indeed, the author seems to accept this lateral shifting as a necessary 

 condition, for he says " Der im Minimum der Ablenkung durchgehende 

 Strahl wird also bei dieser Anordnung nur seitlich etwas verschoben." 



Professor Langley had also quite independently used this particular 

 arrangement of prism and reflecting mirror in a modification of Foucault's 

 u Lifting Prism " for separating different orders of superposed grating- 

 spectra. In his use of it the lateral displacement was recognized as 

 objectionable, and was mechanicalh r corrected for by an ingenious 

 arrangement, designed by Mr. C. T. Child, then assistant in the Obser- 

 vatory, which imparted to the mirror a small angular motion, just 

 sufficient to correct for the angular displacement of the spectral image, 

 as the whole system travelled down through the spectrum. 



