The President’s Address . 
35 
One of our Fellows, Mr. Davis, has recently brought before 
us some highly interesting and novel rotifiers, which he has 
placed provisionally in the genus iEcistes ; and from the dis- 
cussion that ensued when his paper was read, it is evident 
that there is a very useful field for microscopic work in the 
re-examination of the ciliary apparatus of rotifiers with the 
best means of illumination, and with the highest powers that 
can be brought to bear without necessitating a compression 
of the animals to such an extent as would introduce appear- 
ances as fallacious as those which Professor Schjodte has 
dissipated in the case of the louse. 
A paper, published in Germany,* by Dr. Ferdinand Cohn, 
of Breslau, on a series of interesting forms of Infusoria which 
appeared in a marine aquarium, ought to stimulate English 
observers, who have excellent opportunities for adding to our 
knowledge of the microscopic life of our coasts. 
Fresh-water infusoria have been studied much more 
attentively than their marine relations ; but Professor Cohn’s 
paper shows how much may be done with a marine aquarium 
only twelve inches high and twenty inches in diameter. 
The question of iUuminatory apparatus still exercises the 
skill of inventors and constructors. Achromatic condensers of 
various sizes and combinations are now before the public, 
and attention has been called to the different effects which 
result from various proportions in which the central and 
perij)heral rays bear to each other. Combinations of larger 
lenses with longer focal lengths may be made to work with 
the same angle of aperture as smaller lenses with shorter 
focal lengths ; and it is evident that the central rays are a 
constant quantity, while the peripheral rays may be increased 
by larger and suitably made combinations. 
An achromatic condenser intended for research into un- 
known structures should have one stop in which the peri- 
pheral and central rays bear such proportion to each other as 
to facilitate the penetration of the object-glasses, and at the 
same time not to involve too great a sacrifice of surface 
vision. 
The quantities known as penetration and resolution stand 
in necessary contrast to each other ; but as a due proportion 
of angle of aperture to focal length secures a useful combina- 
tion of the two properties in object-glasses, so a due propor- 
tion of the slanting and direct rays brought to a focus by a 
a condenser will help to preserve the balance required for 
most natural history investigations. 
# * Zeitschr. f. Wissenscli. ZooL,’ Bd, xvi, Heft I 860 , 
