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POPULAR SCIENCE REVIEW. 
When mature of this form multiplies by fission for a period extending from 
two to eight days, it becomes peculiarly amaboid, two individuals coalesce, 
slowly increase in size, and become a tightly distended cyst. The cyst bursts, 
and incalculable hosts of immeasurably small sporules are poured out as if 
in a viscid fluid and densely packed j these are scattered, slowly enlarge, 
acquire flagella, become active, attain rapidly the parent form, and once 
more increase by fission. Experiments were next made to determine the 
influence of heat. An ordinary slide containing adult forms and sporules 
covered in the ordinary way was in seven several cases allowed to evaporate 
slowly and placed in a dry heat which was raised to 121°C (260°F). It 
was then slowly cooled, and distilled water was taken up by capillary attrac- 
tion. On examination all the adult forms were absolutely destroyed, and no 
spore could be definitely identified. After being kept moist in the growing 
stage for some hours and watched with the l-50th, gelatinous points were 
seen in two out of the seven cases, which were recognised as exactly like an 
early stage of the developing sporule, and by careful watching these were 
observed to attain the small flagellate state.” The paper is a most valuable 
one, and well merits being read by those who have not yet done so. 
A Mode of rubbing down Needles for Microscopical Work. — At a late 
meeting of the Heading Microscopical Society Dr. Shettle described a 
method of rubbing down needles so as to produce a cutting edge, and yet 
retain the sharp point, by running the needle edgeways through a slice of 
cork, allowing such portion only of the pointed edge to project as it is 
desirable to convert into the knife-blade. The cork, with the needle thus 
inserted, is then firmly fixed in a small hand-vice, the edge of the cork 
being brought to the edge of the vice. The needle should then be laid upon 
a block of metal or other hard material, and rubbed carefully with an oil-stone 
hone, the two sides of the needle-blade being easily produced by inclining 
the vice in a particular manner. The edge of the blade should always (for 
convenience of rubbing) be kept in one direction, and its place determined 
by keeping the needle much nearer one side of the vice than the other. 
The paper also referred to a form of handle, with tapering ferule, by which 
the knife-edged needle is very firmly fixed, and by the use of which a change 
of needle is easily effected. 
Mr. Stephenson's Examination of Diatoms. — This method, which is de- 
scribed and illustrated very fully in the Monthly Microscopical Journal " 
for July, is perhaps the most valuable novelty of the kind that has been yet 
recorded. Mr. Stephenson has conceived of the method of examining diatoms 
(and of course other objects) in solutions of different densities, and watch- 
ing the result. The following paragraph from the paper will give a notion 
of the mode of working, but the original paper must be read by all who 
are interested in microscopy. Mr. Stephensons says : “ If diatoms are ex- 
amined in air, i.e. dry, they are, in some instances, too opaque for trans- 
mitted light, but on immersing them in water, of which the mean index is 
1'336, they become more translucent j with media of higher refractive power 
the translucency increases until the mean index of strong sulphuric acid 
(1434) is attained, in which they become practically invisible. As every 
object which is transparent and colourless becomes absolutely invisible 
when immersed in a colourless medium identical in refractive power with 
