and Energy Distribution of Diffraction Gratings. 537 



at 2*1 //,, 2'1 /jl, and 4'6/u,, due, respectively, to selective 

 emission of the Nernst filament* and to absorption by 

 water-vapour and carbon dioxide in the air ; these will of 

 course appear in any readings which are taken at these 

 wave-lengths, and have nothing to do with the properties 

 of the gratings. 



§3. 



Aii inspection of the set of curves (Pis. VIIL-X.) obtained 

 from any one of the three gratings will show the marked 

 asymmetry of its spectra. A large portion of the energy 

 appears concentrated in one order, or, under the worst con- 

 ditions, in two neighbouring orders. The area under any part 

 of the energy curve is proportional to the total energy thrown 

 into that part of the spectrum. The total area, as well as that 

 for the principal orders of concentration, was determined with 

 the aid of aplanimeter, and the percentage of the total energy 

 which lay in this order was computed from these data. We 

 shall select the Brackett grating No. 10 (PL X.) as the best 

 example, since this combines the two qualities of resolution 

 and concentration to the greatest degree. We append here 

 a table of its properties : 



Brackett Grating No. 10. 



X. 



Order in which 



energy is 



concentrated. 



1-4ii 



1, 

 1, 



1, 



1, 

 1, 

 1 

 1 



1 

 1 



2 



2 

 2 



2 

 2 



2 



3 4 



1-6 



3 



1-8 



3 



20 



21 





2-2 





2-4 





26 





2-8 





2-9 





3-0 



3-2 





34 

 3-9 

 4-5 

 52 

 6-0 



Percent nge of 

 energy in 

 this order. 



32, 18, 



33, 28 

 28, 13^ 



31 



31 



35 



25, 35 



32, 34 



48 



48 



49 



58 



65 



70 



75 



91 



100 



25 

 33 



Starting with the shorter wave-lengths, the energy is seen 



to lie principally in three orders, all on one side of the 



* W. W. Coblentz, Bull. Bureau of Standards, vol. iv. no. 4 (1908). 



