PHENOMENA RELATING TO THE SPECTRA OF HYDROGEN AND HELIUM. 275 
and when these are corrected to apparent breadths, the results agree with observation 
to one-tenth of a millimetre—the observations giving the apparent breadths on the 
right of the main axis. 
The breadths on the left, when measured and corrected to normal dispersion, 
become 
£'i = 5'49 mm., corresponding to y = 12'8, $ x — 4'35, 
^2 = 6*17 mm., „ y = lfO, & = 475. 
Thus 
O- = {(579) 2 —(4'35) 2 —(677) 2 T (475) 8 }/2 (579-677), 
o 
or, on reduction, rr — 4'53 mm. But 14‘66 mm. corresponds to S\ = e = 0'4301 A.II., 
and the separation in wave-length is therefore 
SX = (4'53 x 0730)/l4'66 = 0733 A. U., 
o 
which is very close to Buisson and Fabry’s value 0732 A.IT. Three determinations 
have been made in this way, by the selection of different heights, and the others give 
values SX = 0730, SX = 0734 A.U., the general mean being 07323 A.U. Buisson and 
Fabry’s value is thus confirmed in an absolute manner, even to the third significant 
figure. 
We found it necessary to avoid points too near the base of the photograph, where the 
successive fringes tended to merge into one another by the increase in breadth of the 
parabolas. It was impossible on this account to test the possible existence of a third 
component as given by Sommerfeld’s theory. Its separation would only be O’Ol, and 
its intensity as calculated by Sommerfeld* would necessitate the appearance of its 
maximum in this particular region. 
(XVIII.) The Structure of H,. 
11,3 is also broadened symmetrically in the normal spectrum, and follows the 
probability law of intensity with extreme accuracy. The following are details of a 
typical calculation of its upper portion—-representing the main component—made from 
a larger plate than that quoted for H a . 
For this plate, a. = 0'5 nun., d 0 = 975 mm., and the rate of change of height of the 
maxima per millimetre = 0711 mm. The height of the curve being h u its equation 
when corrected longitudinally should become 
— kx 2 = y—h-L — 0'lllx. 
Co-ordinates measured were, in millimetres, 
2x= 37, 5'3, 67, 7’6, 
y= 14"0, L1'9, 9‘8, 8'0, 
* Loc. cit. 
