GRAPHICAL METHOD OF DRAWING SPECTRA. 21 



The micrometer-scale-readings of 27 air-lines were ob- 

 served in the spectroscope, in addition to those of the ab- 

 sorbing vapour, and taken as abscissae, and the correspond- 

 ing wave-lengths (determined byThalen) 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 six mean points, such that the sum of the 

 shortest distances of the several points of any one group 

 from the curve is zero. 



The mean points were obtained by means of the arrange- 

 ment shown in fig. i . 



The wooden rod A A has a steel point C C fixed at one 

 end, and a graduated scale of tenths of an inch at the other; 

 the axis of the steel point is kept vertical and the scale 

 horizontal while the rod is being rotated, by the strip of 

 wood T T P^ the end P sliding on the table. The centre C 

 being placed at some fixed point, the curve-paper is then 

 adjusted on the table so that some particular division on 

 the scale (found by trial) describes a circle through a group 

 of points, and also through two adjacent groups, one on each 

 side, which circle, if draAvn, would pass not far from the 

 mean point of the three groups we are proceeding to de- 

 termine. The distance from each point of the middle group 

 to the centre C is then read off on the scale to xoVo of 

 an inch, with the aid of a lens and a second scale divided 

 into hundredths of an inch ; and a point plotted near the 

 centre of gravity of the middle group at a distance from C 

 equal to the mean of these readings. 



