ne ‘tie. 
43 
that the second factor is = unity, for I directed the light of 
the sun into the instrument by means of the reflecting part 
of a solar microscope, and not only got measures with extreme 
facility, but had at once a beautiful map of the objective’s 
light-territory. 
‘No.1. As a term of comparison I began with a quarter 
of known excellence. The section was a neat circle nearly 
uniformly bright, surrounded by a penumbra also equally 
well-defined. ‘The measures were taken to 0°005, and they 
gave (denoting their results by A’, and those of the ordinary 
process by A), 
(AME BURN BE fh snk ond: = BOS C8S* 
which for such determination may be reckoned identical. 
«¢ No. 2. The sixteenth just mentioned presented a very 
different appearance. The central part of the section was 
bright, with a jagged outline formed by dark bands spreading 
outwards in the direction of radii very far; so far indeed, that 
T could not trace their end, or see any boundary of the sec- 
tion. Between these bands were stripes of light, but so 
mottled and confused as to satisfy me that this part of the 
illumination could not give distinct vision. Looking directly 
at the objective, light could be seen even at an obliquity of 
85°. The bright part within the radial shadows was not ex- 
actly circular; but, taking an average diameter, I found for 
the part which seemed to me alone likely to give effective 
vision, 
OP ashe ak ACS R60? 
‘¢ No 3. The twelfth, also referred to, is much better: the 
section was toothed all round like a wheel, but to no great 
extent, and a defined boundary was visible. The diameter of 
the bright part gave 
A'=109°,3, 2. . »| A=129°,0. 
ee 
* The compensation of these lenses was adjusted by the Pleurosigma 
Angulata. 
