348 



Mr. T. C. Porter. 



are conveniently expressed by curves (fig. 1) ; along one axis of which 

 the spectrum is plotted, on the other the rate of rotation of the disc. 



H 



\F p\E Soter \DSpectrum\C \B \& A\ 



This last was found from the pitch of the note, obtained by blowing air 

 gently through a known number of equidistant holes pierced in the 

 circumference of the disc, and comparing these notes with those of a 

 set of standard forks. Where the lime-light was used, great pains 

 were taken to keep the intensity of the light constant throughout 

 each experiment, though in these, first experiments the intensity 

 varied for the different curves. It may be well to point out that 

 owing to the nature of the spectrum of lime, the orange is somewhat 

 brighter and the blue and violet darker, relatively, than they are in 

 sunshine. It is for a similar reason, as well as for others too obvious 

 to mention, that the arc light was not employed. It will be seen, 

 that in fig. 1, the intervals of the chromatic scale are plotted in 

 equidistant order, and it should be remembered that the arithmetic 

 increase in the number of revolutions to raise the pitch of the note 

 from C (128 vibrations) to C (256), is only half that to raise it from 

 C (256) to C (512), and so on. The effect of plotting on the axis of 

 Y the actual number of revolutions, would be to make all the 

 curves a great deal steeper. It seems necessary, too, to mention 



