( 731 ) 



Fig. 1. — ^/jo natural size. 



prism must evidently be subjected to an equal rotation with respect 

 to the preceding one, round the points R, Q and J^. Prism 1 remains 

 rigidly connected with the collimator tube ; now let every point of 

 II describe an arc it of a circle round P; then the points of III 

 describe cycloids, those of IV higher cycloids, in which the total 

 rotation of III and IV with respect to the ground plate amounts to 

 2« and 'Sa respecti\'ely, apart from their simultaneous translation. 



In a similar case one of us (F.L.J successfully constructed a 

 toothed-wheel mechanism for a quartz-monochromator years ago, 

 which was now also chosen. The old arrangement for cutting 

 the rather intricate forms of the teeth was now again adopted. An 

 analogous mechanism was, moreover, lately described by Hamy ^) 

 and executed by Jobin. 



For the sake of simplicity prism II is primarily rotated by means 

 of a worm-wheel arc S roughly represented in the diagram, which 

 could be etfected with a bamboo rod from the observer's place. The 

 reading takes place on the circle 6'; the prism tables are provided 

 with german silver feet, which slide oji a glass plate ; prism IV is 



1) M. Hamy, Journ. de pliys. (4) 7, p. 52, 1908; Zeilschf. f. Instr. Kunde 28, 

 p. 122 1<JU8. 



