on the Planet Venus. 
*47 
extended , according to fig. 19, (Tab. XV.) from the proper 
points of the horns a , b, a considerable way, on the edge of the dark 
hemisphere , to d, e; and equally far on both sides, having the 
appearance -of a very dim, constantly decreasing light. But I 
must remark, that in the present more unfavourable situation 
of Vehus, it did not affect the eye as a bluish-grey light, which 
was its appearance March 12, 1790, but only as a dim grey 
light. 
According to my usual projection-measure,* in which each 
decimal line of the projection table is equal to 4" of space, I 
found the apparent diameter of the planet a c b, after repeated 
trials, = 15 lines = 60"; the projection of the crepuscular light 
running into the dark hemisphere ad, b e = 25 lines = 10", 
and fully so, being rather more than less. 
As the crepuscular light could be distinguished from that of 
the points of the horns, by its sensibly fainter colour, I was 
able to measure it from the points. But in order to know 
with certainty whether I had taken the true termination of the 
* In the year 1790, as well as in 1793, I measured this crepuscular light with a pro- 
jection-machine, which is nothing more than a very simple projection-micrometer, 1 
useful in many cases, both by day and night : it gives, for all magnifying powers, the 
measure of the projected object immediately in minutes and seconds of space, without 
the necessity of first measuring a fundamental line. I contrived it for my purpose of a 
selenotopography, and constructed it myself. After an experience of many years, I cer- 
tainly would not lay it aside, in most cases, it being so quick in the use. I have described 
it, in all its simplicity, in my “Beytrdge zu den neuesten astronomischen Entdecknngen ,” 
p. 210, where the older lamp-micrometer of the worthy Dr. Her sc h el is also de- 
scribed before, p. 138, with which this machine may be compared. It has never 
made pretensions to be a new invention, because projection-micrometers of many kinds, 
for example, accompanying microscopes, have long been known. I remember with 
pleasure that, even in the year 1778, the window frames were my projection- micro- 
meter, on which I determined the proportion of magnifying powers to one ano.ther. 
Us 
