[mclennan-dearle] spectrum OF THE MERCUïtY ARC 185 



wooden box lined with absorbent cotton and all the free space between 

 the spectrometer stand and the box was also filled with cotton waste. 

 The box had a window at Si, 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 deflection, 

 when it was again closed. This was repeated from six to ten times and 

 the mean value of the deflection 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 ot this 

 drift the amount of deflection 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 special 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 con- 

 nected 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 deflection was read and the 

 time noted on a stop-watch. The shutter was again closed and after 

 an interval re-opened. The deflection produced and the time corres- 

 ponding 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 backwards to zero time and from the point where it 

 cut the ordinate axis the energy conrtibuted 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 gave the energy contributed by the 

 spectral line. A cooling curve of the type just mentioned is shown 

 in Fig. 5. 



