5 2 Dr. Herschel's Experiments on the Means 
Every thing being now arranged for the measurement, I 
viewed the spurious diameter, with a magnifying power of 
522,7, and compared it to the circles which succeeded each 
other by small differences of magnitude. 
With all the mirror, from the centre to 8,8 inches open, the 
diameter of the spurious disk was ,31 inches. 
With 6,3 inches open, it was less than ,40 and larger than 
> 355 - 
With 5 inches open, it was ,40. 
With 4 inches open, it was ,42. 
With 3 inches open, it was ,465 nearly. 
From these measures it might be supposed that by lessening 
the quantity of light, we bring on a certain indistinctness which 
gives more diameter to the spurious object; to prove that this 
is not the cause of the increase, I used the following apertures. 
With an annular opening from 6,5 to 8,8 inches, the spurious 
disk was rather less than ,18. 
With another from 5 to 8,8 it was exactly ,18. 
With an opening from 4 to 6,5 it was ,22. 
With another from 1,6 to 4 it was ,42. 
(12.) Now since the outside rim from 6,5 to 8,8, which 
reflected less than half the light of the mirror, produced a 
spurious disk less than ,18 in diameter, and the whole light as 
we have seen gave a disk of ,31, it is evidently not the quantity 
of the light, but the part of the mirror from which it is re- 
flected, that we are to look upon as the cause of the magnitude 
of the spurious disks of objects. 
( 13. ) These measures therefore point out an improvement- 
in my former method of putting any terrestrial disk we suspect 
to be spurious to the test. For the inside rays of a mirror, as 
