370 



Mr. G. U. Yule on a Simple 



§ 4. The first analyser on this principle was made for me 

 last summer by Mr. James Hicks. Though suffering from 

 sundry defects (due entirely to my own design) it proved a 

 really useful and workable analyser, but required too much 

 care and patience in use. The construction has now been 

 entirely revised. The present instrument, designed by Mr. 

 Horace Darwin and made by the Cambridge Scientific 

 Instrument Company, is the final outcome. For several 

 suggestions 1 am indebted to Professor Karl Pearson. 



The ruler X X of fig. 1 is a rolling parallel ruler with a 

 rack cut along its front edge (fig. 2). The weight of the 



Fisr. 2. 



rack is counterbalanced by a block projecting from the back 

 of the rule. Normally this block swings just clear of the 

 paper, but it may be held down when one wants to keep the 

 ruler still. 



Corresponding to the disk of fig. 1 we have a series of 

 toothed wheels; the number of teeth in the successive sizes 

 being 240, 120, 80, and so on. Four of these disks have 

 actually been made ; they would probably be workable up to 

 the sixth. The analyser is intended to work to a base-length 

 of 30 centim. ; the rack being cut 8 teeth to a centimetre. 

 The ruler itself is made longer for the sake of stability. 



The largest disk is the simplest. It is a flat disk of brass 

 with teeth cut round the edge. Three windows are cut 

 through it. One, in the centre, is glazed, and the centre of 



