﻿S. P. Langley — Unrecognized Wave-lengths. 



93 



reader's convenience. The limit of precision, imposed by the 

 use of the bolometer, makes it superfluous to introduce any 

 temperature correction, or to give the figures with more exact- 

 ness than we here do. 



Name. 



Grating No. 1 



Grating No. 2 



Grating No. 3 



Radius of curvature 



]626- mm 

 142-1 



202* mm 



146- 



231'0 mm 



l753- mm 

 142-1 



S0-mm 



132- 

 249-l mm 



1627- mm 



Number of lines to mm 



113-7 



Height of ruled portion.. 



"J g •mm 



"Width of ruled portion 



Distance corresponding to 10,000 of 

 Angstrom's units on the line of 

 wave lengths, SiS 2 



133- 

 185-0 mm 







The ruled portion of each of these truly superb instruments 

 occupies from 100 to 150 square centimeters. On their ex- 

 quisite definition we need not enlarge, since sufficient of them 

 are now in the hands of physicists to make our commendation 

 superfluous. 



We have already described the action of the grating. The 

 essential feature, for our purpose, is that, under the stated con- 

 ditions, we can in theory be absolutely sure of the wave-length 

 of the invisible ray under examination by choosing it a multiple 

 of the wave length of some visible line in the superposed spec- 

 trum which is coincident with slit S 2 . 



Thus, in the case of our illustration, we have supposed the 

 sodium line to be used, since this is conspicuous in that of the 

 sun and easily reproduced in that of the arc. The wave-length 

 we are in search of, is always a times the wave-length of D 2 ( a 

 being some aliquot number). In practice we thus, for greater 

 certainty, always form the image of some line in the visible 

 spectrum on slit S 2 , although, as already explained, its mere 

 position on the line Si S 2 is, if the apparatus be in adjustment, 

 a guarantee that none but the exact raj T and its multiple comes 

 under examination. 



Lenses and Prisms. 



The rock-salt lenses, L 1? L 2 , are of different focal lengths for 

 different occasions. For the extremely feeble heat we are con- 

 sidering we are using very clear and perfectly figured salt 

 lenses of 75 mm aperture and 350 mm focus ; this small ratio of 

 aperture to focus in the lenses being required to economize the 

 feeble heat as much as possible. The prism used with them is 

 first set to minimum deviation on some visible line, and then 

 automatically kept there for the invisible ray under considera- 

 tion. 



We owe these specimens of rock-salt to the particular kind- 

 ness of Professor Hastings of Yale College, and their extremely 

 exact surfaces to Mr. Brashear of Allegheny, the maker of the 



