D. P. Todd—The Transit of Venus, 1882. 133 
tween the transit house and the heliostat, for supporting the 
tripod of the engineer’s level to be used in finding the level- 
error of the photographic telescope. The tube of this telescope, 
about thirty-eight feet long and without diaphragms, was 
cut near the middle, and the half near the objective removed. 
A tube was then made, about nineteen feet long, of thin plates 
of iron, diaphragms of sheet lead being inserted as the several 
sections were riveted together. This tube was a half inch less 
in diameter than the original tube, and was slipped inside of 
the remaining half of it—thus giving an air-space between the 
two tubes, in addition to that between the wooden awning and 
the exterior tube. 
ood fifteen inches square was made of card-board to 
cover the plate-holder and the upper part of the pier on which 
it rested. This was blackened inside, and hinged to the wall of 
the photographic house, so that it could be pushed up and out 
of the way when the plates were being put into the holder or 
taken out. The hood was always pulled down before making 
the exposure, and thus the momentary flash of light through 
the photographic house on drawing the exposing-slide, was 
entirely obviated. 
Great care was taken to prevent the mishap of fogged plates 
from any light falling upon the sensitive film other than that 
of the sun from the first face of the heliostat mirror. After 
the adjustments of the heliostat and objective were complete, 
the following test was applied :—a section of thin iron pipe 
two feet long and five inches in diameter was fitted with a stop- 
per at one end, and painted jet black outside and in. The 
sun’s image was adjusted centrally on the reticle-plate, the 
clock-work maintaining it there. The pipe was next set up at 
various points between the objective and plate-holder, the 
stopped end being toward the photographic house. Then the 
pipe being so adjusted that its axis was coincident with the 
axis of the photographic telescope, the eye of an observer 
located south of the plate-holder, and looking north through 
the tube, would readily detect the presence of any object 
— appeared sufficiently luminous to affect the sensitive 
plate. 
The first gi ee of the sun by the wet process—which 
