NOTES AND MEMORANDA. 
207 
ending immediately under the cork. Two series of Liebig’s bulbs, 
charged with pure water, were attached to the two of this arrange- 
ment, one being connected with a large receiver of an air-pump and 
the other left open to the air. The connection between the receiver 
and the adjacent bulb being first cut ofi* by a pinch-cock, the receiver 
was exhausted, and by carefully loosening the pinch-cock a very slow 
passage of the air through the test tubes was secured. The rate of 
transfer was, however, such, that the air above the infusions was 
renewed twenty or thirty times in twenty-four hours. At the end of 
twelve days the turnip juice was perfectly pellucid and free from life. 
Two days’ exposure to ordinary air sufficed to render it muddy. 
After twelve days the pinch-cock was opened so as to allow a 
momentary inrush of the external air, which was immediately checked 
by the reclosing of the cock. Three days afterwards the infusion of 
the test tube into which the air first entered was muddy and crowded 
with life. The contamination did not reach the second test tube. 
Similar experiments completely verify the conclusion, that in Schulze’s 
experiment water may be substituted for sulphuric acid and caustic 
potash without any alteration in the result.^ 
The Ordinary Microscope as a Polariscope for Convergent Light . — 
Professor A. de Lasaulx, of Breslau, describes in the ‘ Bulletin of the 
Belgian Microscopical Society ’ a method of using the microscope for 
this purpose. He points out that the ordinary polarizing instruments, 
whose magnifying power is always feeble, scarcely allow the examina- 
tion by convergent light of small particles or the ordinary micro- 
scopic lamellae of minerals requisite for the petrographic study of 
rocks, so that in the examination of very small crystals it becomes 
difficult to determine the parts most suitable for examination, or to 
grasp a number of optical details. For these observations, however, 
the ordinary microscope with two Nicol prisms is available — all that 
is necessary is to remove the eye-piece, and to work with the 
objective alone between two crossed Nicols. There is then so strongly 
convergent a light that the interference figures can be seen even in 
the thinnest plates. According to the dimensions of the plates under 
observation a high-power objective can be employed, but with the 
higher ones (as Nos. 7 and 9 of Hartnack) the field is not entirely 
round. This can, however, be corrected by applying to the lower 
Nicol two lenses, one of 12 mm. focus, and the other 6 mm., and so 
arranged that each of the two can be used separately or together, 
in the latter case giving a focus of about 5 mm. A completely 
round field is thus obtained even with the highest objectives. • 
By this method it is easy to see the phenomena of interference 
presented with convergent polarized light by crystals, which only 
become diaphanous when they are reduced to an excessive thinness. 
For instance, the figures of substances which exhibit rotatory polariza- 
tion can be seen in thin plates cut perpendicularly to the vertical 
axis, as in cinnabar — also the black cross in the small scales of mica 
enclosed in the thin plates of certain basalts. The employment of 
* ‘ Proc. Roy. Soc.,’ No. 185. 
