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VIII. Observations of the Transit of Mercury over the Disk of the 
Sun ; to which is added , an Investigation of the Causes which 
often prevent the proper Action of Mirrors. By William 
Herschel, LL. D. F. R. S. 
Read February 10, 1803. 
Th e following observations were made with a view to attend 
particularly to every phenomenon that might occur during the 
passage of the planet Mercury over the sun's body. My solar 
apparatus, on account of the numerous observations I have lately 
been in the habit of making, was in great order for viewing the 
sun in the highest perfection ; and, very fortunately, the weather 
proved to be as favourable as I could possibly have wished it. 
The time at which the observations were made, not being an 
object of my investigation, is only to be considered as denoting 
the order of their succession. 
November 9, 1802. About 40' after seven o’clock in the 
morning, I directed a telescope, with a glass mirror of 7 feet 
focal length, and 6,3 inches in diameter, to the sun ; and per- 
ceived the planet Mercury. It was easily to be distinguished 
from the openings in the luminous clouds, generally called spots, 
of which there were more than forty in number. Its perfect 
roundness would have been sufficient to point it out, had I not 
already known where to look for it. 
io h o'. When the sun was come to a sufficient altitude to 
show objects on its surface with distinctness, I directed my 
