of ascertaining the Magnitude of small celestial Bodies . 49 
me an opportunity for experiments on spurious disks ; to 
obviate this inconvenience, I used small drops of quicksilver. 
They are more lucid, and will give a bright spot with very 
little sunshine. Many of these drops of all sizes were exposed 
upon a plate of glass, and some on slips of steel. The manage- 
ment of them is a little different from that of the globules. 
For in order to represent a double star these must be placed 
one almost behind the other, as otherwise they cannot be 
brought near enough without running together. The following 
general observation will include all the necessary particulars. 
The bright spots on drops of quicksilver are very small 
compared to the size of the drops. 
They are not proportional to the magnitude of the drops, 
though less on the small ones and greater on large ones. 
In some of the large ones the bright spot is about ~ or -T- 
of the diameter of the drop. 
The magnitude of the luminous spots is liable to changes, but 
is rather more permanent than with the silver globules. 
There is a little difference in the colour of the luminous 
spots ; they are generally of a brilliant white, but sometimes 
they incline to yellow, and the small ones to ash-colour. 
With high magnifying powers they are very well defined, 
and, on account of their brightness, will bear these powers 
better than the silver globules. 
If M and rn, stand for the diameters of the large and small 
mirror of my telescope, then will an aperture = y^ r™ ~f~ m % 
give half the light of the telescope. With this I examined 
two of the drops, and found the luminous spots upon them 
with 
MDCCCV. 
H 
