54 Dr, Herschel’s Experiments on the Means 
magnitude ; to resolve this question, I viewed them first with 
the inside rays of the mirror, then with the outside, and found 
that in botli cases the distance of the limes remained vithout 
the least alteration. I viewed them also with the whole mirror 
open, but it occasioned no change. 
21 st Experiment. Measures of the comparative Amount of the 
spurious Diameters, produced by the Inside and Outside Rays. 
I divided the aperture of the mirror into two parts, one 
from o to 4,4 and the other from 4,4 to 8,8 inches. When I 
measured the spurious diameter of a globule, the inside rays 
made it ,40; with all the mirror open it was ,31 ; and with 
the outside rays it was ,22., 
( 14. ) From this we may conclude that the diameters given 
by the inside rays, by all the mirror open, and by the outside 
rays, are in an arithmetical progression ; and that the inside 
rays will nearly double, the diameter given by the outside. It 
remains however to be ascertained whether this will hold good 
with spurious disks of various magnitudes. 
It will not be necessary to carry the divisions of the aperture 
farther ; for as the application of these experiments is chiefly 
intended for astronomical purposes, we can hardly do with less 
than half the mirror open ; and on the other hand with a very 
narrow rim of reflection from the outside of the mirror, dis- 
tinctness would be apt to fail. 
o.Q.d Experiment. Trial of the Criterion on celestial Objects , 
I viewed aLyrae with the outside rays, and found its spurious 
disk to be small ; with all the mirror open it v/as larger, and 
with the inside rays it was largest. 
