82  S& Newcomb on observing the coming Transits of Venus. 
ments employed in observing the transit of Venus, in order to 
determine what correction should be applied to the observa- 
tions of one to make them comparable with those of the other. 
It would be a comparatively simple operation to erect an artifi- 
cial representation of the sun’s-disk at the distance of a few hun- 
dred yards, and to have an artificial planet moved over it by 
clockwork. The actual time of contact could be determined b 
electricity, and the oe of the planet and the dis 
y actual measurement. With this apparatus it would be easy 
to determine the personal errors to which each observer was lia- 
ble, and these errors would approximately represent those of the 
observations of actual transit. 
till, it would be very unsafe to trust entirely to any deter- 
But, although we cannot determine contacts by photography, 
istance 
want is the value in arc of an inch on the photograph plate. 
This di ination is not without difficulty. It will not do to 
trust the measured diameters of the images of the sun, because 
they are affected by irradiation, just as the optical image is. If 
the plates were nearly of the same size, and the ratio of the diam- 
eters of Venus and the sun the same in both plates, it would be 
safe to assume that they were equally affected by irradiation. 
But should difference show itself, it would not be safe to as- 
sume that the light of the sun encroached equally upon the dark 
