ZOOLOGY AND BOTANY, MICROSCOPY, ETC. C)69 
Position of the Light-filter in Photomicrography.* — Since 1866 it 
has been the generally received doctrine, says Dr. R. Neuhauss, that the 
position of the filter for producing monochromatic light is of the greatest 
importance. This doctrine, laid down by Moitessier and followed by all 
other writers, states that the maximum of absorption is attained when the 
filter is placed before the collecting lens, and its minimum when inserted 
between the lens and its focus. 
By experiments with a yellow disc placed in the position of the object 
on the stage and using an ordinary non-orthochromatic silver-bromide- 
gelatin dry plate covered with a silk-paper sensitometer in the one case, 
and inserting the yellow disc between the light and the lens in the other, 
it was found that the two images were exactly alike in every respect. 
For both the exposure was exactly 15 minutes, and in both negatives the 
numbers could be read when the layers of silk paper were not more 
than sixteen. 
Similar results ensued from using a layer of a saturated solution of 
picric acid 3 mm. thick. Hence it is quite indifferent whether the filter be 
placed near the lens or its focus. 
(6) Miscellaneous. 
The Microscope in Geology. — A course of twelve lectures on the 
Microscope in Geology (with special reference to the structure and origin 
of the stratified rocks), is now being delivered by Professor H. Alleyne 
Nicholson in the British Museum (Natural History), Cromwell Road, 
on Mondays, Wednesdays, and Fridays, at 3 p.m., beginning 6tli October 
and ending 31st October, 1890. Admission to the course is free. 
/3. Technique.! 
Cl) Collecting Objects, including Culture Processes. 
Cotton-wool as a substitute for Silk in Bacteriological Work.J — 
Dr. E. Braatz finds that animal products have a much greater affinity 
for mercury than vegetable, and for this reason advises that cotton-wool 
threads be used instead of silk threads in bacteriological work. 
Effect of highly concentrated Media on Bacteria. § — Prof. H. 
Buchner replies to Metschnikoff’s assertion that the inhibitive influence 
of the body fluids on micro-organisms is to be ascribed to the greater con- 
centration of these fluids. The author first remarks that the germicidal 
property of serum is quite extinguished by heating it to 55° for half an 
hour, although its degree of concentration remains quite unchanged. 
He then gives the results of experiments made with highly concentrated 
media, viz. blood charged with 23 per cent., and also with 40 per cent, 
of cane sugar. In both instances, although there was at the very 
* Zeitsch. f. Wiss. Mikr., vii. (1890) pp. 20-2. 
t 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. 
X Centralbl. f. Bakteriol. u Parasitenk., viii. (1890) pp. 8-9. § T. c., pp. 65-9. 
