26 On Syncheta Mordazx. a Le 
Apertures. Raaate: Apertures of 
Dry Objectives. } MRED { Immersion Objectives. 
(Nascent Pencil.) 
re) re) i ° i 
50 32 44 386 40 
80 50 50 58 
120 70 32 80 
170 83 14 96 (nearly.) 
Considering, therefore, the enormous difficulties encountered in 
absolutely correcting the oblique and eccentrical pencils of an objec- 
tive, and how much their defects are diminished by reducing the 
aperture, the extraordinary difference between the immersion and 
dry objective as regards the aperture required to transmit the same 
actual divergent pencil radiating from the particle in the balsam, 
we need not be astonished at the comparative ease and cheapness 
with which foreign opticians construct really good immersion water- 
lenses. 
(To be continued.) 
V.—On Synchxta Mordax. By C. T. Hupsoy, LL.D. 
Puare LVI. 
THREE years ago I found S. mordaxz swarming at Christmas time in 
a pond near Exmouth on the road to Budleigh Salterton, and though 
I have frequently since met with specimens I have never till lately 
found it in sufficient numbers to make it worth while to renew my 
attempt to make out its structure. However, in the beginning of 
this March I captured some in a clear pond at Portbury, and by 
April they had so multiplied in the pond that every dip of a two- 
ounce bottle would bring up a score of them. They were not to be 
found everywhere in the pond, but kept mainly to one spot which I 
noticed was sheltered by hillocks from the prevalent west wind, and 
in which the water crowfoot was growing. Throughout April they 
swarmed, but since then their number has been steadily decreasing ; 
and now it requires an hour’s search to catch a dozen, the pond all 
the while remaining apparently unaltered. I had hoped, both here 
and at Exmouth, to have found the males under such favourable 
circumstances; but I was disappoimted. If I had caught one I 
think I should have noticed it, as I know the male of S. tremula. 
Synchata has had but scant justice done it. All the drawings 
I have seen (except Gosse’s of the mastax) give only the vaguest 
hints of its internal structure, and represent it from only one (and 
that invariably the same) point of view ; whereas it should be drawn 
from at least three points of view to convey an accurate idea of its 
shape. The difficulty of obtainmg under the microscope any view 
of Syncheta except that usually given is very great. It is the 
swiftest swimmer for its size among the Hydatinea, as well as the 
