Emission Spectrum of the Mercury Arc. 689 



gave the energy contributed by the spectral line. A cooling 

 curve of the type just mentioned is shown in fig. 5 

 (PL XV.). 



The intensity of the radiation of any particular wave-length 

 as measured by the spectrometer was found to vary with 

 small displacements of the lamp or scale, and so all measure- 

 ments were compared with the deflexion produced by the 

 radiation from the green line X = 5461 A.U. This line, which 

 was of strong intensity, possessed 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 radia- 

 tion and that it affords good definition in all parts of the 

 spectrum. Against this, however, 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 the 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 

 work with a fairly wide slit. On this account, one cannot ex- 

 pect in working with a rock-salt prism to reach the precision of 

 wave-length measurement attainable with a grating spectro- 

 scope, or to differentiate between lines very close together 

 with the same facility as with a grating. The prism, how- 

 ever, enables one to obtain a reliable register of the maxima 

 in the energy spectrum, and these can be used as a guide for 

 finer measurements with an instrument such as a grating. 

 The following table shows the width of spectrum covered in 

 the different ranges by the thermopile slit for a slit- width of 

 1 mm. 



These results are also shown graphically in fig. 6. Although 

 from this table there seems to be a considerable width of 

 spectrum covered by the slit, still it must be remembered 

 that since in all our measurements readings were taken at 

 intervals of 0*1 fi, the energy curves could be filled in 



