20 Wadsworth — Design for large Spectroscope Slits 



be of interest. In tig. 1 is shown a rear view of the slit the 

 back plate being removed to show the mechanism clearly. 

 The two slit jaws slide between guides screwed to the front of 

 the slit plate and each has a lug extending through this plate 

 and projecting about l cm behind it. To the right hand jaw is 

 screwed a [] shaped bar c?, the side arms of which pass around 

 the ends of the slit and the center of which is opposite the 

 lug on the left hand jaw. This lug is tapped to receive a 

 screw on which are two threaded portions, one engaging with 

 the thread in the lug and the other of just one-half the pitch 

 of the first engaging in a nut h which is screwed to the slit 

 plate. When therefore the screw is turned, say to the right, 

 the jaws are separated by an amount equal to the motion of 

 the point of the screw -with -inference to the threaded lug, 

 say a distance a?, wdiile the whole jaw system is drawn to the 

 right by the action of the fixed nut h a distance equal to ix 

 The center of the slit therefore remains fixed, the jaws open- 

 ing out from it. A spring bearing against the lug on the left 

 hand jaw provides for the return motion and takes up all back 

 lash in the screw. The graduated head gives by its motion 

 over the graduated drum the whole number of turns and frac- 

 tions of a turn enabling the width of slit to be determined at 

 a glance. It will be noted further that the accuracy of separa- 

 tion depends only on the accuracy of the screw which works 

 in the slit lug, the screw at A serving only to keep the jaws 

 centered. As the motion is positive in one direction only (on 

 opening), as in the case of a single motion slit, there is no 

 danger of injuring the edges of the slit by turning the screw 

 too far. 



Further and perhaps most important of all in a slit of this 

 size, the thrust of the screw on both jaws is central and there 

 is consequently no tendency to twist the jaws in their guides. 

 The new slit has been constructed by Grunow, who has as 

 usual admirably executed the work. A front view of the new 

 slit is given in fig. 2 and to show its compactness and also its. 

 great size as compared with the usual spectroscope slits, two of 

 these each having a clear opening of 2 cm and representing the 

 two types already described are shown at its side. 



Astro-Physical Laboratory, Smithsonian Institution, 

 Washington, D. C, March, 1894. 



