071 a New Automatic Motion for tlie Spectroscope. 10 L 

 in this disk having for their middle lines the curves 



,.F(f), ,.F©, ,..(!), ,.F(f) 



between the same limits of r. The other disk can turn on the 

 axle K ; its initial line is K T ; and a a' , h U , c c' , d cV are slits 

 similar to A A^ B B^, C C^ D D^ K P, K Q, K R, K S are 

 arms capable of turning on the axle K ; and p p' ^ q (]' , r /, ss' 

 are slits in these arms directed towards K. 



Now let P, Q, R, S be the intersections of the slits in the 

 disks, and let the arms be turned so that their slits pass over 

 the points P, Q, P, S. Then, if pins are inserted in the slits 

 at these points, perpendicularly to the disks, the arms will be 

 retained in such positions that the lines K P, K Q, K R, K S 

 will divide the angle between K 0, K T into five equal angles. 

 The first prism is placed on the line K and fastened to the 

 fixed disk ; the next four are placed on the lines K P, K Q, 

 K P, K S and are fastened to the corresponding arms ; and 

 the last prism (half of which is replaced by a reflecting prism) 

 is placed on the line K T and fastened to the moving disk. 

 All the prisms are placed at the same distance from the centre ; 

 and it is obvious that they v^ill have the motion required for 

 the spectroscope, the angle between the position of consecutive 



prisms being always ^• 



In order to keep the pins perpendicular to the disks, I have 

 a pair of fixed and a pair of moving disks, the slits in each pair 

 being exactly parallel, and the moving disks being rigidly 

 connected together j each arm also consists of two parallel 

 arms rigidly connected. The arrangement is shown in a sec- 

 tion of the instrument given in fig. 2. 0^ are the fixed 

 disks, TT^ the moving disks, K K^ the axle, kpk' p' an arm 

 carrying one of the prisms (which is shaded), P P^ one of the 

 pins going through the arm and all the four disks. The pins 

 are prevented from dropping out by nuts screwed on at each 

 end. 



The telescopes are fastened to the fixed disks and are shown 

 in fig. 4. X is the collimator with the slit, Y the observing- 

 telescope, and Z the collimator with the micrometer. The 

 position of the micrometer and the form of the prism opposite 

 the observing-telescope is explained in the Philosophical Ma- 

 gazine for April 1876, '^ A New Arrangement for the Micro- 

 meter of the Automatic Spectroscope," fig. 3. Part of each 

 moving disk has to be cut away to allow space for the tele- 

 scopes and fixed prism. This can be done, without removing 

 any useful part, in the way shown in figs. 3 and 4. 



