455 
1909-10.] Mapping of Grating Spectra. 
lies in the second order ultra-violet. The intensity ratios of the components 
at once enables these orders to be ascertained. 
When employing the method herein described, it is advantageous to 
have the total effect of the quartz in the optical path about that indicated. 
In this way lines whose vibrations are about octaves show the greatest 
dissimilarity of intensity ratio in the photographs. The differentiation of 
orders is then most easily accomplished where the first and second orders 
overlap, and where the need for differentiation is greatest. There is also 
.... * . : . ■ . ; ■ : ..J 
Fig. 4. — Differentiation of Orders. 
a sufficiently great difference in the intensity ratios at the part of over- 
lapping of the second and third orders. As a rule, there is no call for 
differentiation tests between higher orders. 
It is of course well known that some lines show abnormalities in the 
distribution of light intensity in the various components. It is seldom, 
however, that the intensity ratios are so abnormal as to cause a risk of 
error in employing the method which has been described. 
The method seems well adapted for use with gratings of large dispersion. 
Not only does it effect an enormous saving in expenditure when work is 
being carried out on the rarer elements, but the second photograph obtained 
with the Zeeman effect is of considerable intrinsic importance, and may be 
VOL. XXX. 30 
