46 
of 0:125 diameter, which cuts off the irregular part. With 
this the Angulatum was seen in the most satisfactory man- 
ner; but when it was replaced by one carrying a central 
disc of 0°125, which stops out the good part, it would 
scarcely show even a trace of lines on the coarsest Pleuro- 
sigmata. 
«¢ These facts, I trust, are sufficient to show the necessity of 
attending not merely to the amount of aperture, but also to 
its quality. The objectives which I have examined are all 
of first-rate excellence in the good part of their apertures : 
I have not named their makers, nor the friends to whose 
kindness I am indebted for the power of examining so pre- 
cious a collection, because it might lead some to unjust con- 
clusions respecting the merits of the former. It is from no 
wish to depreciate the debt of gratitude which we owe them, 
or to undervalue the wonderful skill which they have shown 
in correcting aberrations so perfectly as they have done in 
these very objectives, that I make this communication, but 
from a wish to point out to them a defect which they will 
be able easily to remedy, and which at present occasion- 
ally interferes with their complete success. What its cause 
is can only be determined by one familiar with the construc- 
tion of the objectives, but it probably arises from some of 
the lenses being so small that their edges meet the lumi- 
nous pencil and reflect false light. I am induced to suspect 
this from observing that when in No. 6 the first lens was 
separated from the others by a revolution of the compensating 
screw beyond its proper adjustment (in which state, however, 
it would have no definition), the radial shadows disappear 
altogether, and A’ = 110°,3. Reducing the separation by a 
fourth of the revolution, they are seen, but of no great length, 
and enveloped in a blue zone, which surrounds the bright 
centre, the latter giving A’= 139°,9. ‘This increase of dis- 
tance has the effect of stopping out the margins of the other 
