[MCLENNAN- DEARLE] SPECTRUM OF THE MERCURY ARC 



187 



work with a fairly wide slit. On this account one cannot expect in 

 working with a rock salt prism to reach the precision of wave-length 

 measurement attainable with a grating spectroscope or to differentiate 

 between lines very close together with the same facility as with a 

 grating. The prism, however, 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 



WAVt LltlGTH m /i. 



Fig. 6. 



covered by the slit, still it must be remembered that since in all our 

 measurements readings were taken at intervals of 0-l;u, the energy 

 curves could be filled in between these readings and the wave-lengths 



