360 
NOTES AND MEMORANDA. 
freezing could be ensured and a large number of sections obtained 
before the tissues became loosened from the freezing chamber. It 
should be remembered, that to avoid the expense and incumbrance of 
a special condensing apparatus we have to provide for the free evapo- 
ration and subsequent condensation of ether in the same chamber, and 
consequently a sacrifice of about one-fourth the bulk of ether used is 
sustained. I regard as the requisite of a good freezing microtome the 
following conditions : — 
1. The instrument should be of the greatest possible simplicity. 
-2. The freezing should be rapid and expeditious. 
3. The metallic constituents should be such as to retard thawing 
of the tissue when once frozen. 
4. A minimum of ether should be expended. 
Now, I would claim for my instrument a fulfilment of these con- 
ditions as far as is possible, without the employment of an exhausting 
and condensing apparatus ; and I would on these grounds advocate 
its use by those who require a most satisfactory microtome, and one 
comparatively inexi^ensive. The first condition, that of simplicity, is 
too self-evident to dwell upon ; the second also is ensured, as the 
tissue is frozen in less than twenty seconds, whilst it remains ad- 
herent to the cover of the freezing chamber for a period sufficing for 
the cutting of* a dozen sections or more. A very small quantity of 
ether suffices for freezing, and three-fourths of its bulk becomes con- 
densed, and may be collected in a bottle attached to the tube (e). I 
used a graduated bottle for the ether spray, and can thus read off the 
amount of ether expended in freezing. The costliness of ether has 
been urged against its employment in ordinary section cutting by 
one authority of note, and the objection would prove of great weight 
in case a large amount of ether was lost at each operation ; it is but 
necessary, however, that the instrument be once seen in good working 
order to dispel all such notions from the mind, as the results obtained 
are of the first order, and the expenditure of ether very trivial.” 
The paper includes a description of the method of cutting sections 
of brain, but space prevents any further extract here, and the original 
may be profitably referred to. 
Angular Aperture defined . — Professor Romyn Hitchcock, of New 
York, brought this subject before the Indianapolis Congress. In 
order that the term “ angular aperture ” should mean something 
definite, and to avoid ambiguity and misunderstanding in future dis- 
cussion on the subject, he proposed to adopt a definition of the term 
which, right or wrong, should be recommended to the microscopists 
of the country as a convenient and uniform usage. The triangle 
method was proposed for general adoption, considering the angular 
aperture of a microscoj^e objective to be the angle of the apex of a 
triangle having a base equal to the available diameter of the front 
lens, and a height equal to the actual focal length (working distance) 
measured in air for a dry lens, and in the fluid employed for an im- 
mersion lens, the collar being adjusted for the most perfect definition 
in every case. 
While nearly all the members seemed to be personally in favour 
