166 MEMORANDA. 
At the date of that publication both Dr. Curtis and myself 
believed the markings on Pleurosigma angulatum to be hexa- 
gonal, the photographs on which this opimion was based being 
magnified, originally about 1000 diameters, and afterwards 
enlarged to 7300 in a copying camera. Subsequent observa- 
tions with higher powers, however, have satisfied us that this 
opinion is erroneous, and that, as, in fact, Mr. Wenham had 
previously suggested, the real conformation of the markings 
is circular. 
The photographs on which this opinion was based were 
some of them made with an objective of one fiftieth of an inch 
focal length, constructed by Powell and Lealand, some of them 
with a Wales’ objective of an eighth of an inch focal length, 
corrected for photography as above indicated, and the neces- 
sary amplification being given by the introduction into the 
draw-tube of an achromatic concave also corrected for photo- 
eraphy. No eye-pieces were employed. With either of these 
arrangements Dr. Curtis obtained direct photographs of 
‘ excellent definition and powers, varying with the distance up 
to 2500 diameters, with about three feet distance, beyond 
which, in either case, the pictures began to diminish in 
clearness. To obtain any given power, it was found the 
one-fiftieth required a few inches’ greater distance than the 
one-eighth and amplifier. The use of an eye-piece, or of a 
concave amplifier similar to that used with the one-eighth, but 
of much lower power, was carefully tried with the one-fiftieth, 
but it was found that the results were not well defined, so 
that 2500 diameters must be regarded as the maximum power 
to be obtained photographically with the one-fiftieth. With 
the one-eighth and amplifier the same power was attained 
with perfect ease. The negatives taken in this way were 
readily enlarged in the copying camera to 19000 diameters. 
I enclose albumen prints of the pictures with both powers, 
and by both glasses, for comparison. The enlarged prints are 
almost fac-similes of those of about 2500 diameters ; that by 
the one-fiftieth is perhaps a trifle sharper, but it was acciden- 
tally taken with 200 diameters less than were allowed the 
one-cighth. The flatness of field is, of course, greatest im the 
one-eighth picture. 
It might be suggested that the eighth being photographi- 
cally corrected, had the advantage over the fiftieth in this 
comparison; but the photographic correction for an eighth is 
already small, and that for a fiftieth may be regarded as too 
trifling to modify the results very greatly. . 
In both the small pictures and the large you will notice 
the markings are perfectly circular spaces, which in the small 
