of ascertaining the Magnitude of small celestial Bodies. 51 
deceptions, I fixed them up against a tablet 154 inches from 
the eye, where it was intended to project the spurious disks of 
the globules, and examined them at that distance with the 
naked eye. Comparing then the size of the black to the white, 
I judged No. 1 of the black to be a little larger than No. 6 of 
the white circles. By a measure taken afterwards, it appeared 
that the black one was ,40 and the white ,39. Without sup- 
posing that every estimation may be made at this distance with 
equal accuracy, to the hundredth part of an inch, it is suffi- 
ciently evident that no material deception can take place in 
estimating by either of the sets of circles on account of their 
colour. 
17 tli Experiment , with different Illumination. 
A similar experiment was made in the microscope, by which 
the globules were measured. Two of them were placed on the 
measuring stand, and with an illumination from below, they 
appeared black, and were projected on white paper. The dia- 
meter of each globule and the distance between them were 
then measured. After this, I caused the illumination to come 
from above, and the globules being now of a silvery white, 
were projected on a slate. In this situation, when I repeated 
the former measures, no difference could be perceived. 
iSth Experiment. Measures of spurious Disks. 
The spurious disk of a globule w r as then projected on the 
tablet where the white circles were placed. While I was com- 
paring it with No. 4, which is ,31 in diameter and estimated it 
to be a little less than the circle, the spurious disk grew brighter; 
but it remained still of the same size ; so that a variation in 
the quantity of the illumination will make no difference. 
H 2 
