105 
The curve consists of arcs of circles, which pass through 
the six mean points, with radii varying from 64 J inches at 
the upper end, to 61 § inches at the lower end, drawn by 
means of the rod AA, having a drawing pen fixed to it. 
The wave-lengths corresponding to the scale-readings of 
the observed lines in the two spectra, were then found by 
measurement from the curve to ^oVo th of an inch, by means 
of a lens, and scale of Tooths of an inch. 
The above method of graphically drawing a curve, may 
be extended to the case where the points are nearly 
uniformly distributed, and not divided into groups naturally 
as in the case of the air-lines. 
Divide the points into any number of groups, about an 
equal number in each group. Find the mean points as 
described, by aid of lens and vernier scale on the rod ; draw 
the curve through the points by the best practical method. 
The number of groups taken will be guided by judgment, 
and will depend on what degree the proposed curve shall 
be. The greater the number of groups, the greater will be 
the degree of the equation to a curve passing through their 
mean points, and the greater probably will be the irregular- 
ities in the variation of the radius of curvature. 
Construction of Beale for Map of Spectrum. 
The arrangement shown in Fig. 2 has been employed to 
draw the map scale, and found to be an accurate and ex- 
peditious method. 
