on the Planet Venus. 
215 
the effect of the extended illumination, and in his angle of 
15° 19' will amount to more than two degrees and a third. 
His measures are also defective ; as probably the mirror of his 
7-feet reflector, which was a very excellent one, was by that 
time considerably tarnished, and had lost much of the light 
necessary to shew the extent of the cusps in their full bril- 
liancy. 
I do not give the calculations I have made of the extent of 
the twilight of Venus, because my measures were not so sa- 
tisfactory to myself as I wish them to be ; nor so near the 
conjunction as we may hereafter obtain them ; neither were 
they sufficiently repeated. My computations, however, when 
compared to those given in the paper on the atmosphere of 
Venus, shew sufficiently that it is of much greater extent, or 
refractive power, than has been computed in that paper. 
Those calculations indeed are so full of inaccuracies, that it 
would be necessary to go over them again, in order to com- 
pare them strictly with my own, for which at present there is 
no leisure. 
I ought also to take notice here, that the same author, it 
seems, has taken measures of the horns of Venus by an in- 
strument, which, in his publications, he calls a projection table , 
and describes as his own * ; of which, however, those who do 
not know its construction may have a very perfect idea, when 
they read the descriptions of my lamp, disk, and periphery 
over 44 of a great circle ; and, in the situation which he mentions, that is, perpendi- 
cular to the line of the cusps at the time of the greatest elongation, and when the ap- 
parent diameter of Venus is 60", (as he makes it) it must measure o",3§4. 
* See Beitrdge zu den neuesten astr onomiscben Entdeckungen, p. 210. And Seleno- 
topographische Fragmente, p. 63. 
2 F 2 
