ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
415 
which gives perfectly reliable results, it is necessary that makers should 
have accurate values of the equivalent focal length of eye-piece and 
objective stamped on their mountings, and also the tube-length stamped 
on the body-tube. 
A standard tube-length should be agreed upon. The author considers 
that of 180 mm. of Continental makers more convenient than the 10 in. 
generally adopted in England and America. The upper limit of the 
tube-length should be the focal plane in which an image would be formed 
by the objective if there were no field-lens. In a negative eye-piece this 
plane is midway between the diaphragm and the optical centre of the 
eye-lens. Eye-pieces should therefore be so constructed that when 
slipped into position this plane should be exactly at the top of the body- 
tube. Such par-focal eye-pieces have been made for several years past 
by the firm of Zeiss. The lower limit of the tube-length should be the 
point within the objective which behaves as an optical centre. The 
distance from the top of the body-tube to the extremity where the 
objective is screwed on is taken a little shorter than the desired tube- 
length, say 160 mm. instead of 180 mm. Then in the formula — } — : 
P p 
= y,p' = 180 and p can be calculated, since /is known. Subtracting 
then from p the working distance between slide and the exposed lens, 
we have the distance within the objective of the point which acts as an 
optical centre. Allowance can then be made in the mounting of the 
objective to make this point just 20 mm. from the extremity of the body- 
tube where the objective is screwed on. 
R. Technique.* 
Arloing, S. — Cours elementaire d’anatomie generate et notions de technique 
histologique. (Elementary Course of General Anatomy and Histological 
Technique.) Paris, 1890, 8vo, 388 figs. 
Behrens, W. — Leitfaden der botanischen Mikroskopie. (Outlines of Botanical 
Microscopy.) Braunschweig (Bruhn) 1890, large 8vo, 288 pp., 150 figs. 
Bonnet, R. — Kurzgefasste Anleitung zur mikroskopischen Untersuchung thierischer 
Gewebe. (Concise Introduction to the Microscopic Examination of Animal 
Tissues.) Miinchen (Rieger) 1890, 2 figs. 
Paul, F. T. — On the relative Permanency of Microscopical Influence of the different 
Staining and Mounting Agents. Liverpool Med.-Chirurg. Journ ., X. (1890) p. 65. 
Cl) Collecting Objects, including Culture Processes. 
Method for making Permanent Cultivations.f — Herr W. Praus- 
nitz preserves roll and puncture cultivations (and even liquid ones 
provided the liquefaction is not too general) by filling the tubes with a 
gelatin solution to which a disinfectant has been added. The tubes 
are placed in ice water, the cotton wool plugs removed, and the fluid and 
antiseptic gelatin solution is then slowly poured in through a pipette. 
The tube is then plugged with a cork cut off flush with the top, and finally 
* This subdivision contains (1) Collecting Objects, including Culture Pro- 
cesses ; (2) Preparing Objects ; (3) Cutting, including Imbedding and Microtomes ; 
(4) Staining and Injecting; (5) Mounting, including slides, preservative fluids, &c. ; 
(6) Miscellaneous. 
f Centralbl. f. Bakteriol. u. Parasitenk., ix. (1891) pp. 131-2. 
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