214 F. ADDEY ON THE MEASUREMENT OF DRAWINGS. 



divisions of PQ will then be one of 50 diameters' magnification, 

 and so on for the other scales. 



Thus the whole rectangle is used twice over. 



To avoid confusion it is convenient to mark the figures for 

 this second series of scales in a difierent-coloured ink from that 

 used to mark those for the first series. 



A photograph of an actual scale made in this way is shown in 

 Fig. 2. On this scale lines have been drawn for magnifications 

 1,000, 950, 900, 850, etc. (or 100, 95, 90, 85, etc.). For intermediate 

 magnifications it is easy to judge where the corresponding line 

 should be drawn, and the edge of a ruler, or of a sheet of paper, 

 may be placed parallel to the top of the diagram in the proper 

 position to give the scale desired. This is better than loading 

 the scale with a large number of lines. 



The sloping lines beyond the diagonal of the rectangle are 

 obtained by extending the division of any convenient one of 

 the lines parallel to AB in Fig. 1, beyond the point at which the 

 line chosen is cut by the diagonal, by the use of dividers, and 

 drawing straight lines through the points so obtained from the 

 bottom left-hand corner of the rectangle to the side corre- 

 sponding to BC in Fig. 1. The scale numbers along the top 

 line are continued down this side. As an example, the line 

 ST in Fig. 1 has been dealt with in the manner just described. 



The side of the rectangle corresponding to AD in Fig. 1 is 

 ten centimetres in length. This is a convenient size, as the 

 divisions for the even tens' or hundreds' magnifications are then 

 Bpaced one centimetre apart. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XIV., No. 87, November 1921. 



