24) ON A GRAPHICAL METHOD OF DRAWING SPECTRA. 



The divisions of the map-scale are drawn at Z Z, with a 

 very thin drawing-pin, very close to the edge of the set 

 square ; so that the variation of the inclination of the draw- 

 ing pin is a very inappreciable error. If the map-scale is 

 required to be longer than the scale S, another very fine 

 perpendicular ink line L' L' must be drawn at an exact mul- 

 tiple of 10 millimetres distance. 



To prevent mistakes in drawing the principal divisions, 

 the line L L is arranged so that they may be drawn when 

 the corresponding principal divisions on the scale S S are 

 coincident with the line. 



The advantages of this method of drawing a map-scale 

 arise from the fact that the error due to the observation of 

 the coincidence of two lines is much less than that of plot- 

 ting a point opposite each division of the millimetre-scale — 

 also in the fact that the error due to drawing a line with 

 a thin pen very close to the edge of the set square (the pen 

 approximately at the same inclination) is much less than 

 that of drawing the divisions through the plotted points, 

 judging by the eye. 



If only one scale be required on the map, place the edge 

 of the scale S near to the line KK. If two are required, 

 place the edge of S S halfway between KK, K'K', to lessen 

 the errors due to imperfections in the straight edge of the 

 T-square. 



Construction of a Vernier Scale for the above Map-Scale. 



The drawing-board, T-square, set square, and paper as 

 in fig. 2. OO (fig. 3), a strip of paper on which the vernier 

 scale is to be constructed, its edge being parallel to T T; the 

 scale S S is attached to the edge of the set square, and slides 

 with it, by means of the pins and elastic N N as before, but 

 inclined to the edge of TT at an angle a^=sec~' |-i; it is kept 



