Wadsivorth — Design for large Spectroscope Slits. 19 



Aet. IY. — A new Design for large Spectroscope Slits ; by 

 F. L. O. Wadsworth. With Plate I. 



Of the various forms of double motion spectroscope slits 

 which have been designed, the two forms in most common use 

 are the parallel ruler form, as fitted to most German instru- 

 ments, and that form in which the jaws slide in guides and 

 are moved simultaneously in opposite directions by a right and 

 left hand screw. The first form is very convenient but re- 

 quires very careful work in order to secure accurate results 

 and is moreover apt to stick if the working parts are not kept 

 clean. Moreover the jaws must be made somewhat longer 

 than the available width of slit and the plate upon which they 

 are mounted must be still larger to admit of room for mount- 

 ing the levers, etc., etc., so that the' whole arrangement be- 

 comes somewhat bulky in the case of a large slit. Another 

 difficulty lies in determining the exact width of slit from the 

 reading of the screw which is used to open it. The second 

 form mentioned avoids all these difficulties except the last, 

 but introduces others of a different character. As the screw is 

 necessarily at one side of the jaw a twisting strain is thrown on 

 the latter by the friction of the opposite guide tending to 

 make the slit of unequal width at the two ends, and as the 

 motion of the jaws is positive in both opening and closing 

 there is danger of injuring the edges of the slit by screwing 

 them too tightly together. For the same reason and also be- 

 cause there is no spring to take up the lost motion in the two 

 screws, the zero point of the screw reading is uncertain and it 

 is therefore difficult to determine the exact width of the slit 

 from the reading of the screw. Prof. Langley had previously 

 designed on this plan a very large slit with jaws 5 cm in length 

 for his bolometric work in the infra red solar spectrum, in 

 which the above difficulties were reduced to a minimum, by 

 the accuracy and care with which the mechanical work was 

 done, but which nevertheless showed after some usage a decided 

 bluntness and want of parallelism of the edges. 



As the size of the slit increases these difficulties of course 

 grow more serious. When it became necessary therefore to 

 provide a new and still larger slit having jaws with 10 cm clear 

 opening, I proposed a new design which I had previously used 

 with advantage in a double motion micrometer, in which the 

 thrust of the screw on both jaws is central. As it appears to 

 be new and as it possesses several advantages over either of 

 the two forms previously noted a brief description of it may 



