134 D. P. Todd—The Transit of Venus, 1882. 
tion of the plate-holder being duly changed and recorded be- 
tween the several photographs of each set. Before the plate- 
holder was secured in its final position, each photographer had 
critically inspected all these plates; and without conferring 
with any one else had made a memorandum of the numbers o 
those plates in each set which he regarded as indicating the 
best focus. In addition to this, nearly all the trial-plates were 
examined independently by Captain Floyd and Mr. Fraser, and 
all of them on two separate occasions by myself. Nearly all 
the different determinations were, when collated, in surprising 
agreement; and the adopted setting of the plate-holder could 
not have been in error by so much as the ;,4;;th part of the 
focal length of the objective. In point of fact, the position of 
the focal plane was definitely indicated to the ;,'5sth part. 
The superior definition of the photographs of the transit was 
an entire compensation for all this trouble. 
The first photograph of the transit of Venus was taken at 
19" 11™, local mean time. The exposure was 14° long, and the 
slit 3'°-0 wide. Only a very faint image came out on the 
plate. The fourth exposure, somewhat shorter, and with the 
slit the same width, at 19" 17™, gave a picture sufficiently in- 
tense for measurement; but the vertical diameter of the sun 
was something like #* shorter than the horizontal one, and the 
limb was not well defined. Plate No. 13, at 19® 50", slit 
1-0 in width, and exposure 0*-4 long, is the first photograph 
of real value, though the five immediately preceding it may be 
worth measuring. The width of the slit was gradually reduced 
as the altitude of the sun became greater, being successively 
0-75, 0-5 and 3", until at 21" 20™, it was set ata width of 
0'"-25, and was so kept until the end. The exposures were 
quite uniformly 0%-25 in length. 
ree records of the times of exposures were kept—one au- 
tomatically on the chronograph, the circuit being broken at the 
precise instant when the middle of the slit passed the central 
vertical line of the reticle-plate; a second record, by myself, 
taken from the mean time chronometer in the dark room, and 
set down at once in the photographic record ; and a third record, 
kept in the transit house by Psd Floyd, assisted by Mrs. 
Fraser, the approximate time of the exposure-click of the chro- 
nograph-armature being taken from the face of the sidereal 
chronometer. As neither the observation of the mean-time 
chronometer time of the exposure, nor the automatically re- 
corded sidereal-chronometer time, could properly be regarded 
as complete without the corresponding number of the photo- 
graph, this latter was, in every instance, made equally a matter 
of observation with the time itself; and no plate was ever 
exposed until I had myself seen and recorded the number it 
