212 



p. ADDEY ON A UNIVERSAL SCALE FOR 



to deal with greater magnifications than this, the same principle 

 can be applied to form a miiversal scale having a range of magnifi- 

 cations as great as may be desired. 



The principle on which the scale is constructed is shown in 

 Fig. 1. AB is a centimetre scale of any convenient length, the 

 first centimetre of which is divided into millimetres. Since 

 one micron is 0-001 millimetre, each millimetre will represent 

 one micron magnified 1,000 times. Thus, this scale may be 

 used to measure a drawing made to a magnification of 1,000 



2 MM. 



3 MM. 



4 MM. 



5 MM 



Fig. 1. 



diameters, each millimetre on the drawing representing one 

 micron on the actual object. 



A rectangle ABCD, of any convenient depth, is drawn on the 

 line AB, and straight lines are drawn from every division of the 

 scale AB to the angle D. For clearness the lines from the milli- 

 metre divisions are omitted in Fig. 1. The whole rectangle 

 is thus divided up into a series of triangles, having a common 

 side AD, and bases of varying length. 



By bisecting the common side AD in P, and drawing a line 

 PQ parallel to the line AB cutting DB at R, we have, by similar 



triangles, :p^ = -^ppj. Thus, since DP is half DA, PR 



DA AB 



half AB. 



