[GILLIS] THE VARIATION OF WATER VAPOUR LINES 47 
The experimental arrangements were as follows:— 

A 
Eira. 1. 
Sunlight was reflected from a heliostat H to a mirror M, both of 
which stood in a room by themselves. Thence the light passed through 
a pipe PP and slit S to a grating G, and was then reflected to the 
eye-piece E. The grating stood in a room which could be completely 
darkened, and, moreover, in order to shut out all extraneous light, the 
arm GE and the grating were covered with black cloth. 
The usual adjustments having been made, the original intention 
was to take observations by a photographic method, a camera being 
mounted at E instead of the eye-piece. However, this was abandoned 
in favour of a modification of C. 8. Cook’s method. The eye-piece was 

Nive. Be 
provided with a sleeve S carrying a single silk fibre F. This was 
moved in and out by means of metallic springs SS that bent over a 
millimeter scale attached to the outside surface of the tube. When 
the fibre is placed near the focus, one very dark line is seen in the ap- 
proximately monochromatic light from the grating; however, when it 
is gradually moved away, two dark lines of less intensity appear. 
These are the diffraction bands, which are caused by the passage of 

