ON THE TOTAL ECLIPSE OF THE SUN, MAY 28, 1900. 
405 
Part IV.— The Prismatic Cameras. 
By W. J. S. Lockyer, M.A., Ph.D., and A. Fowler. 
Description of the Disfruments. 
The instrument employed by Dr. Lockyer was that employed in India by Mr. 
Fowler, the aperture being 6 inches, and focal length 7 feet 6 inches, and two 
prisms ol 45° being fixed in front of the objective. A different series of exposures, 
however, was arranged, and unlike those made in India, each was made on a separate 
plate, so that proper treatment in developing could be given. The spare part of the 
mirror was utilised for a finder, as in India, with which observations of the cusps for 
giving signals before totality were adso made. 
The instrument employed by Mr. Fowler consisted of a G-inch Taylor triple 
objective, of focal length 20 feet 3 inches, with the 9-inch prism of 45° Ijelonging to 
the Solar Physics Observatory. The camera tube was a skeleton one, similar to that 
described in Mr. Payn’s report. The lens and prism were provided with a substantial 
mounting of steel and brass, which rested on a brick pier, and the camera, of solid 
construction, was placed at the proper distance on another brick pier. Two plate 
holders were provided, each holding five plates of size 15 inches by 3 inches. The 
back of the camera was an extended one, so arranged as to protect all the plates 
except the one which was l)eing exposed through an opening of the same size as the 
plate. The moon’s disc as represented on the photographs is 2^ inches in diameter, 
so that a small margin of the plate was available in case there should be a slight 
error in centering the spectrum. A finder was arranged to view the sun or stars by 
reflection from the first surface of the prism. It may be mentioned that in all the 
plate holders employed, the use of springs was discarded, the plates being held in 
position by india-rubber pads arranged to give even pressure on the edges. This 
change was considered desirable in consequence of certain slight departures from 
perfect focus in some parts of the spectra photographed in India, which were 
attributed to the bending of the plates by the central springs of the usual form. 
The 20-foot lens was received so shortly before the expedition left England, that it 
was only possible to make a rough trial of the instrument before it was set up at 
Santa Pola. 
Both prismatic cameras were worked in conjunction with siderostats, calculations 
having shown that the position angles of contact were favourably situated after 
reflection. 
In the case of each instrument the necessary assistance was rendered by men from 
the “ Theseus.” One man made the exposures at the prism end at pre-arranged 
signals from Dr. Lock yer or Mr. Fowler at the camera end ; another recorded the 
times at which the exposures were made, the beginnings being marked by the signals 
