182 REPORTS FROM THE SECTIONS. 
(180°) is the same as the Tolles, but its balsam angle of 108° is in 
excess of the latter by 8°. I am by no means clear about the 
real meaning of “balsam angle,” only that ceteris paribus greater 
balsam angle gives greater resolving power. 
In the trial of these glasses I used the following tests for 
oblique light :—P. angulatum S. gemma, P. fasciola and Nitz- 
chia sigma, mounted dry, and Amphiplewra pellucida in balsam, 
illumination by an ach. condenser of 160° angle, Wenham’s reflex 
in immersion contact with slide by means of glycerine, and the 
concave mirror only. I was quite prepared to find the oil lens a 
diatom-smasher, and fully expected it to beat the Tolles by oblique 
light, and I must admit that the German giant beats the American, 
if they were race horses I should say by about half a length. 
On angulatum with the deepest eye-piece at my command (a p)I 
could see no différence, both glasses behaving splendidly, and 
giving such brilliant definition under the D eyepiece that it seemed 
Nitzchia Sigma, dry. This slide is by far the most difficult 
test object (to me) I possess, and until I got the Tolles 75th resisted 
all my efforts. I tried P. and L.’s {th and Zeiss’ 4th, and 
Zeiss’s 3th, 4th, and 34; on it without seeing the ghost of 
a marking ; with the Tolles I the striz with 
ce 
mersion lenses, it goes far beyond any other mode of illumination 
_ On the above tests I used the Tolles with water amare a 
now put the slide of pellucida (in balsam) on the stage, 
