unrecognized Wave-lengths. 159 



been good enough to execute them for us of the very short 

 focus and open ruliDg necessary for our particular work. 



Let us designate them as grating No. 1, No. 2, and No.3. 

 The dimensions of grating No. 1 have been given in a prior 

 memoir, but we repeat them here with some other data, for 

 the 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 

 exactness than we here do. 





Grating 

 No. 1. 



Grating 

 No. 2. 



Grating 

 No. 3. 



Radius of curvature... 



1626 mm. 



142-1 

 102 mm. 



146 



2310 



1753 mm. 

 1421 

 80 mm. 

 132 



2491 



1627 mm. 

 113-7 

 75 mm. 

 133 



185-0 





Width „ 



Distance corresponding to 10,000 of 



Angstrom's units on the line of 

 wave-lengths S x S 2 



The ruled portion of each of these truly superb instruments 

 occupies from 100 to 150 centim. On their exquisite defini- 

 tion we need not enlarge, since sufficient of them are now in 

 the hands of physicists to make our commendation super- 

 fluous. 



We have already described the action of the grating. The 

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

 conditions, 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 spectrum 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 B 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 ad- 

 justment, a guarantee that none but the exact ray and its 

 multiples come under examination. 



Lenses and Peisms. 



The rock-salt lenses L x L 2 are of different focal lengths for 

 different occasions. For the extremely feeble heat we are 

 considering, we are using very clear and perfectly figured 



