VISIBLE AND NEAR-VISIBLE RADIATION 



151 



illumination. In a well-constructed instrument, this usually amounts to 

 about 1 per cent in the vicinity of a strong region of emission, i.e., spectral 

 line. That is, a background of intensity where there should be no radi- 

 ation will exist to about 1 per cent of the intensity of the strong line. In 

 instruments constructed of fused material, containing striations, this 

 impurity may be many times greater. For most critical work, a double 

 monochromator should be used (see Fig. 6(6)), the radiation from one 



4000 A 3000 2000 



Fig. 7. — Comparison of spectra, using single and double monoohromators at the 



same slit width. Full line shows single monochromator. Dotted line shows double 

 monochromator. 



monochromator passing through a second monochromator, the second 

 slit of the first monochromator serving as the first slit of the second. 

 Where no change in slit width is made, the scattered light will then be 

 reduced to one-fourth of its original value. The total transmission of 

 the instrument now becomes 16 to 25 per cent, but in a properly con- 

 structed double monochromator, the resolution (purity) will be doubled 

 without change in slit width. Figure 7 (57) shows the observations of 

 mercury-arc spectrum with one monochromator, and the dotted line 

 shows the spectral distribution as determined with two identical instru- 

 ments combined into a double monochromator, the slit widths being 

 unchanged. 



