406 
SIR NORMAN LOCKYER AND OTHERS 
to expose, and the ends by certain calls from the time-keeper at the echpse clock. 
In each case also a bluejacket handed the dark slides as they were recpiired, and 
another placed them in their bags when the exposures had been made. 
In the case of each siderostat a spare portion of the mirror was used to reflect 
light into a camera with a transmission grating in front of the lens, these instruments 
being worked by bluejackets. 
The instruments were roughly focussed in the first instance by observing the 
sharpness of the edges of the spectrum when the light of the sun was reflected into 
them, and afterwards by taking photographs of the spectrum of Arcturus. In the 
case of the 20-foot, considerable difiiculty was experienced with the focussing, for the 
reason that the focus determined by a star did not hold good for the sun next day; 
this was especially noticed on the day of the eclipse, when it was further found that 
between early morning and eclipse time, the focal length, as judged by the sun’s 
edge, was shortened by nearly 2 inches. The stellar focus was accordingly disre¬ 
garded, and an attempt was made to obtain the focus by observing on the ground 
glass the Fraunhofer lines given by the disappearing crescent just before totality. 
It was then found that the range of adjustment was insufficient to allow perfect 
focussing, and there was no time to make the necessary alteration in the length of 
the tube. The photographs consecpieiitly lack the perfection of definition which had 
been hoped for, but they nevertheless give a good deal of information, as will appear 
later. The cause of the variation of focus has not yet been investigated, but it is 
recalled that in a general wa}^ the focus appears to have dejDended to some extent on 
the hour angle at which the siderostat mirror was used. 
Table of Exposures. 
It was intended that the exposures in both instruments should be as nearly as 
])ossible alike. The actual times in the case of the 20-foot prismatic camera were as 
follows:— 
Number 
of plate. 
Time of exposure. 
Duration. 
Remarks. 
hr. 
min. 
see. 
1 
4 
12 
36 
Inst. 
2 
4 
12 
38 
Totality commenced, 4h. 12m. 38s. 
1 3 . 
4 
12 
40 
4 
4 
12 
42 
5 
4 
12 
44 to 
6 secs. 
4 
12 
50 
6 
4 
12 
58 to 
32 „ 
4 
13 
30 
1 7 
4 
13 
34 to 
11 „ 
4 
13 
45 
8 
4 
13 
47 
Inst. 
9 
4 
13 
49 
10 
4 
13 
51 
Totality ended, Ik. 13m. 53s. 
