4.80 REPORT—1904. 
3. Some Results with the Solar Physics Observatory Photo-Spectro-Helio- 
graph. By Wii1u1amM J. 8. Lockyer, M.A,, Ph.D., PRAS., 
This paper contained a description of the spectro-heliograph and the results 
which have been produced with it. 
The complete instrument for taking photographs of the sun in monochromatic 
light consists of three parts: a siderostat, a lens for throwing the solar image on 
the primary slit, and the spectro-heliograph, 
The first carries a plane mirror of 18 inches in diameter and has electric slow 
motions which are operated at the spectro-heliograph. The lens, a Taylor photo- 
visual, has an aperture of 12 inches and a focal length of 18 feet. The spectro- 
heliograph moves horizontally in a direction at right angles to the incident beam 
on the primary slit. It consists of one triangular frame moving on another 
triangular frame, the former rolling on three balls supported by the latter. This 
movement is operated by a falling weight, and controlled by a piston moving in an 
oil chamber. On the movable frame is fixed a double tube, at the extremities of 
which are fixed the two 4-inch Taylor photo-visual lenses and the two slits. The 
spectrum is formed in the plane of the secondary slit by means of a 6-inch plane 
mirror and a 6-inch prism of 45° angle. Nearly in contact with the secondary 
slit, but independent of its motion, is placed the plate-carrier. The diameter of 
the solar image on the primary slit is 23 inches, and this is also the size of the 
monochromatic image at the secondary slit. The dispersion is such that the 
length of spectrum from F—K is 1°62 inches. 
The secondary slit is so adjustable that the ‘K’ line of calcium can be com- 
pletely isolated, and this slit is also so curved that the line can be isolated through- 
out its whole length. 
Up to the present time the ‘K’ line has alone been utilised, and, whenever 
possible, photographs have been secured of the ‘ K’ radiations on the disc and those 
round it. In the case of the former, with a summer sun and untarnished mirrors, 
good pictures can be secured in 15 seconds, but this time has to be considerably 
prolonged during the other months of the year. 
In the case of the ‘K’ radiations round the disc, or the prominences, under 
similar weather conditions, 15 minutes is required for a full exposure. These 
pictures are obtained by placing in front of the primary slit a metal disc equal in 
diameter to the solar image. 
The photographs exhibited showed numerous pictures of the disc taken during 
May, June, and July of the present year, all of which showed fine detail and 
surface mottling. Several composite photographs, that is, pictures showing the 
solar disc and limb photographed on the same plate but consecutively, were also 
exhibited. 
Attention was also drawn to the very rapid changes which the prominences 
on the limb underwent in comparatively short intervals of time. Thus, on July 14 
a prominence in the north-west quadrant of the sun in an interval of one hour 
changed from 160,000 miles in length to 96,000 miles, while its height increased 
from 50,000 to 60,000 miles in the same time. 
Another instance occurred on July 19, when an enormous prominence, 192,000 
miles in length, grew to 216,000 miles in five hours, At the same time its height 
changed from 55,000 to 60,000 miles. 
At the present time the instrument is being employed to obtain, as far as is 
possible, a daily record of the ‘K’ radiation on the disc and the prominences on the 
limb; but it is hoped, as soon as a sufficiently large grating can be secured, to 
investigate the distribution of other substances. 
4, On the Unsymmetrical Distribution of Rainfall about the Path of a 
Barometric Depression. By Hueu Rosert Mitt, D.Se. 
Heavy rains are usually divisible into those accompanying thunderstorms and 
those accompanying ordinary cyclonic disturbances. In ten cases of the latter 
