688 Prof. McLennan and Mr. Dearie on the Infra-red 



the spectrometer-stand and the box was also filled with cotton- 

 waste. The box had a window at S 1? covered by a shutter, 

 and a second window as well, through which to read the 

 wave-lengths on the drum. As an additional precaution an 

 asbestos screen was always placed between the lamp and the 

 spectrometer. The lamp itself gave rise to certain errors 

 due to variations in the current, and to the occasional deposit 

 of a drop of mercury on the face of the tube from which the 

 radiation was taken. These latter errors were sufficient at 

 times to produce false maxima of considerable magnitude. 

 In taking all readings, the drum was set at the desired wave- 

 lengths and the shutter was opened until the galvanometer 

 reached its maximum deflexion, when it was again closed. 

 This was repeated from six to ten times, and the mean value 

 of the deflexions was taken as a measure of the energy in the 

 particular wave-length selected. Zero drift was always con- 

 siderable, on some days amounting to as much as 140 mm. in 

 readings extending over the space of an hour. To eliminate 

 the effect of this drift the amount of deflexion on opening 

 the shutter was read, and also the distance which the spot of 

 light returned on closing the shutter. The mean of these 

 two was then taken as the correct reading. When every 

 imaginable precaution was taken, it was still found that 

 maxima appeared in the energy curves which apparently did 

 not represent spectral lines. However, it was possible, by 

 repeating the readings over any given portion of the spectrum 

 on different days, to differentiate between true and false 

 maxima and so to identify the spectral lines. In taking the 

 measurements, it was necessary to distinguish between the 

 energy which was contributed by a spectral line and that 

 which was contributed by the continuous spectrum due to 

 the radiation from the heated quartz of the lamp itself. To 

 -do this, a circuit breaker was connected in series with the 

 lamp, the drum was adjusted to give the wave-length of the 

 desired line, and the shutter was opened immediately after 

 the circuit was broken. The ensuing deflexion was read and 

 the time noted on a stop-watch. The shutter was again 

 closed and after an interval reopened. The deflexion pro- 

 duced and the time corresponding to it were again noted. 

 In this way several readings were taken, and a cooling curve 

 was plotted from them. This curve was then extended back- 

 wards to zero time, and from the point where it cmt the 

 ordinate axis the energy contributed by the radiation from 

 the hot quartz of the wave-length under investigation was 

 ascertained. This reading was subtracted from the reading 

 taken when the lamp was in operation, and the difference 



