TRANSACTIONS OF SECTION A. 



467 



all without stopping, water power unfortunately not being available in the place 

 where the machine has been erected. 



Up to the time when this machine was commenced, no diffraction grating had 

 been ruled larger than 2 inches square, and but few as large as this. The new 

 machine was designed to rule 6 inches square, but can if required rule 6| x 6^. 

 Professor Rouland's machine, which seems to have given such good results in America, 

 will, the author believed, rule 6" x 4''. 



In describing his ruling apparatus it will be convenient to take the various 

 parts in the following order : — (1) The screw and nut ; (2) The platform which 

 supports the grating ; (3) The tool carriage ; (4) The means employed for turning 

 ■the screw. These four divisions constitute the ruling machine proper ; besides these, 



Fig. 1. 



however, there is, on a separate support, (5) the driving machinery which works the 

 ruling machine, performing all the necessary operations automatically. 



Fig. 1 is a diagrammatic plan of the ruling machine and driving machinery. 



I. A perfect screw of any considerable length is a thing which, it has been 

 considered, is almost impossible to produce. 



A screw in general has four kinds of errors which are easily measui-able. (1) 

 Eccentricity or want of straightness in the axis ; (2) Gradual variation of pitch 

 extending over considerable lengths ; (3) A periodic variation of pitch of the same 

 length as the pitch of the leading screw of the lathe in which it was cut; (4, 

 which is perhaps the most important), A periodic error which recurs at each thread. 

 In dividing engines, as ordinarily made, the nut is rigidly attached to the platform 



H H 2 



