60 



Prof. J. Trowbridge on Spectra of 



lever LjM is outride the dark box, and the operation of ex- 

 posing the plate and closing the camera can be performed 

 without opening the box. 



Flo-. 2. 



A 



o 



I o. 



The method which Rowland used in mounting the grating- 

 is undoubtedly preferable to the above, W'hen an accuracy 

 greater than one-tenth of an Angstrom unit is aimed at. Tne 

 advantage, however, of the method I have employed is in its 

 compactness, and in the possibility of working, in a light 

 room ; moreover^ in gas-spectra an accuracy even to one 

 Angstrom unit is often respectable. 



Fig. 3, PI. ly., (the photographs are reproductions of nega- 

 tives, and are not positives) is a reproduction of the spark- 

 spectrum of calcium in the neighbourhood of the H.H lines of 

 the solar spectrum, taken by the method of successive dis- 

 charges of known amounts of electrical energy. The discharges 

 ran from one to five. It is interestino- to notice on the neoatives 

 that the photometric intensity of the lines estimated by the 

 blackness is not directly proportional to the amount of energy. 

 Thus the spectrum produced by four discharges is not twice 

 as intense as that produced by two discharges. It is notice- 

 able, also, that the calcium lines wave-lengths 3737 and 3706 

 are stronoer on the neo-ative than those which coincide with 

 the H.H lines, and always appear with these lines. 



Fig. 4 (PL IV.) is a negative of discharges running from 

 one to four in a Geissler tube of olass which did not contain 



