Resonance Spectra of Sodium Vapour. 585 
in the red region of the spectrum have been already 
published *. 
The resonance spectra obtained by stimulating the vapour 
with the radiations from the cadmium and zinc quartz arc- 
lamps have been photographed with a large grating, and the 
wave-lengths of the bright lines which form the series have 
been determined to within a tenth of an Angstrom unit, a 
tremendous step in advance over the work recorded in the 
earlier paper, in which we could not be sure of the wave- 
lengths to within less than about 2 Ang. units. 
Many new sources of monochromatic light have been used, 
and the prism spectrograph improved by the addition of 
a new long-focus photographic objective by Cooke. The 
resonance spectra described in the previous paper have been 
photographed over again with this improved apparatus, and 
we now know the wave-lengths of the lines to within a little 
less than one unit. During the present year they are being 
photographed over again with a 12-foot concave grating 
with a collimating lens of six feet focal length. This work, 
which will take an enormous amount of patience, is being 
carried on in collaboration with Mr. Felix E. Hackett, 
Fellow of the Royal University of Dublin. 
Familiarity with the preceding paper above referred to 
will be presupposed in presenting the more recent results, as 
the details and methods have been already fully described. 
A few improvements have been made in the apparatus. 
Owing to the long exposures it has been found necessary to 
put a heating-coil and thermostat in the large three-prism 
spectrograph. This keeps the temperature constant to 
within a tenth of a degree, and improves the definition in no 
small degree. The telescope-lens has been removed and 
replaced by a Cooke photographic objective of 3-J inches 
aperture and 40 inches focus, which was made to order. 
This lens was corrected so as to be as nearly as possible 
achromatic for the spectrum range 4500-5700, within which 
the resonance spectra, which are being studied, fall. It also 
gives fair definition over the range of the entire visible 
spectrum. The wet cotton jackets on the sodium tube have 
been replaced by tightly wound coils of lead pipe (5 mms. 
diameter) through which a current of cold water circulates. 
In working with open air arcs much difficulty was experi- 
enced from the wandering of the arc over the surface of the 
electrodes. The image of the arc on this account moved 
* " On the existence of Positive Electrons in the Sodium Atom/ 
Phil. Ma<?. February 1908. 
