OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
5 
placed in slots running through the length of A. The light had therefore to pass 
through these two glasses before it reached the paper beneath them. 
Experiment has shown that the aperture can be completely uncovered in less than 
i^th of a second, and closed in the same time ; hence no appreciable error in exposure 
arises from this cause when the exposure lasts several seconds. The paper is placed 
on a grid, running the whole length of the box behind the flexible shutter, the bars 
falling between the aperture and different portions of the paper are exposed by 
moving the block A into different positions. These positions are indicated by a 
scale engraved on one side of the frame, a mark engraved on the shutter being 
moved successively to each division of the scale. As there are ten divisions of 
the scale, it will be seen that forty diflPerent exposures can be obtained on the same 
paper, ten through each aperture. The frame is closed by a wooden back, lined with 
velvet, which is pressed against the paper by means of a couple of brass springs 
working against buttons fixed to the frame. 
XXVIII .—Comparison of Results. 
In order to get a scale of blackness, by which to measure the intensity of light 
acting, it became necessary to have some means of exposing similar paper to light of 
different known intensities. From the blackness so produced a curve of blackness 
could be constructed, and the blackness produced by the sun with different exposures 
could be read off. Now, increase in time of exposure is equivalent to increase in 
intensity of light acting, at all events, when the exposures are as prolonged as those 
