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THE ROYAL SOCIETY OF CANADA 



The intensity of the radiation of any particular wave-length as 

 measured by the spectrometer was found to vary with small dis- 

 placements of the lamp or scale and so all measurements were com- 



TinL \n 5tce>. 



Fig. 5. 



pared with the deflection produced by the radiation from the green 

 line A = 5,461 A°.U. This line which was of strong intensity pos- 

 sessed the advantage of being practically outside the region of the hot 

 quartz radiation. 



III. Accuracy of Measurements. 

 In work on infra-red spectra the means generally employed to 

 produce the spectrum are the prism and the grating. The latter has 

 the great advantage that it absorbs but little radiation and that it 

 affords good definition in all parts of the spectrum. Against this, how- 

 ever, is the fact that the energy is divided up into several orders, that 

 these orders often overlap in the infra-red, and that the distribution 

 of energy in any one order does not always correspond with the true 

 distribution in the spectrum. The prism, on the other hand, gives 

 but one order, so that a maximum of energy is found in each and every 

 wave-length. It limits the measurements, however, to the region 

 where the radiation is transmitted without absorption. With prisms 

 of rock salt the radiation is transmitted up to wave-lengths of 60,000 

 A°.U.; but it is difficult to secure good definition in the longer wave- 

 lengths. This difficulty, moreover, is enhanced by the fact that in 

 order to secure sufficient energy in the weaker lines it is necessary to 



