78 The Total Solar Eclipse of 1900. 
of a prismatic camera, placed in a horizontal position, and fed by a flat mirror 
placed in front of the prism. 
The object-glass of the telescope was of 4-in. aperture, and about 4 ft. focal 
length, and was corrected for the photographic rays. A dense flint prism, of 
about 88° refracting angle, and a circular silvered-glass mirror, of 12 inches 
aperture, were employed. 
The peculiarity of the instrument consisted in the apparatus for obtaining a 
continuous succession of spectra, as described in Sir Howard Grubb’s Report 
above. 
As it was intended that the exposures should not exceed 2 or 3 secs., we 
considered that, with the moderate dispersion of the prism used, it would be 
unnecessary to apply any movement to the 12-in. mirror during the short period 
for which each plate was exposed. 
It would have been possible, of course, to place the fixed mirror so as to 
reflect the image of the Sun into a horizontal telescope in any azimuth. But 
there was one point of the horizon which possessed an important practical advan- 
tage over any other. It was essential that the rod, or plunger, to which the 
mirrors were attached, should move up and down in a vertical direction, and of 
course the refracting edge of the prism had to be placed parallel to this rod. It 
was also essential that the refracting edge of the prism should be parallel to the 
tangent to the Sun’s image at the point of contact, or as nearly so as possible. 
In order, therefore, that the prism should be used with its edge vertical, it was 
necessary to arrange the apparatus so that the diameter of the Sun’s reflected 
image, which passed through the point of contact, should be horizontal. It was 
therefore necessary to find the particular position im which the camera should be 
placed, so that these conditions might be fulfilled. 
This is found in the following way :— 
Let HAMS’ represent the horizon, HZ7B 
the meridian, S the position of the Sun, Z the 
Z 
zenith, and let ASB be the direction of that a B 
diameter on which the given point of contact , 
lies. Then the angle ZSB (= V) is “‘the angle De 
from the vertex,” measured in the direction 
NESW which may be computed from the 
Nautical Almanac. Also, ZS is the zenith dis- 
tance of the Sun (=¢) and BZS is the Sun’s 
azimuth (= A). the. @. 
Draw the great circle AW bisecting the angle SAS’, and take AW equal to 5 ; 
join SW, and produce it to meet the horizon in 8’. Then it is clear that S’ is the 
