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Ordinary Meeting, March 21st, 1876. 
Edward Schunck, Ph.D., F.R.S., &c., President, in the Chair. 
Dr. Arthur Schuster exhibited an interesting collection 
of objects brought by him from Siam and the Western 
Himalayas. 
“ On a Graphical Method of Drawing Spectra,” by Mr. 
William Dodgson, Whitworth Scholar. Communicated by 
Professor Roscoe, F.R.S., &e. 
Construction of Curve for ascertaining the wave-lengths. 
A number of points are first plotted on the curve-paper, the 
abscissae of these points being the micrometer scale-readings 
of certain lines, as observed directly in the spectroscope, or 
being absolute— or relative measurements taken from an 
existing photograph ; whilst the ordinates of these points are 
the corresponding wave-lengths, taken from the determina- 
tions of some reliable observer. 
If there be a large number of points taken, the curve ought 
not to be drawn actually through them all, or it would 
probably be irregular ; it therefore becomes necessary to 
draw a mean curve, i.e., a continuous even curve which shall 
pass near to all the given points, some being on one side 
and some on the other. 
The following method was employed to determine the 
wave-lengths of the well defined edges of the bands in two 
absorption spectra. 
The micrometer scale readings of 27 air-lines were obser- 
ved in the spectroscope, in addition to those of the absorbing 
vapour, and taken as abscissa ; and the corresponding wave- 
lengths determined by Thalen as ordinates. These 27 points 
when plotted on the curve-paper, fell naturally into six 
groups ; a mean point was then found for each group by 
accurate measurement, and the curve drawn through these 
Proceedings — Lit. & Phil. Soc. — Vol. XV. — No, S. — Session 1875-6.. 
