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POPULAR SCIENCE REVIEW. 
then readily set out in the field by simply placing other ranging-rods in line 
with these several images. This could be done by looking through the 
unsilvered half of the half-mirror, and planting the rods opposite to and over- 
lapping the successive reflections. No error could arise in the manipulation, 
and the whole process of setting out a true curve was shortened and simpli- 
fied. After setting the mirrors to the requisite tangential angle, no further 
adjustment or support was needed than could be afforded by the top of the 
ranging-rod placed at the commencement of the curve, and shifted occa- 
sionally to any stake on the curve that the limits of distinct vision might 
require.” 
How to Obviate the Concentration of the Heat-rays in Solar Observations. — In 
examining the sun with telescopes, the concentration of the calorific rays renders 
observation unpleasant and often dangerous. This unsatisfactory condition 
has, however, been done away with by means of a device adopted by M. Leon 
Foucault, who has conceived the idea of utilizing the property which certain 
metals have of absorbing heat-rays, whilst allowing the merely luminous 
ones to pass through. Silver, when deposited by a particular chemical pro- 
cess, in very thin layers, possesses this property in a very high degree. M. 
Foucault has sheathed the objective of a telescope with a layer of this metal, 
and there is produced at the focus of the instrument an image perfectly clear 
and agreeable to the eye. It exactly resembles one which a violet-coloured 
glass would produce.— Tide The Reader. 
ZOOLOGY AND COMPAEATIYE ANATOMY. 
The Habits of the Lemming. — The habits of this creature ( Lemmus Nor- 
vegicus) have been described in a paper by M. Guyon, read before the French 
Academy. M. Guyon kept the animal alive from the 15th of August, 1863, 
to the 18th of June, 1864, when it was accidentally killed. The Lemming 
was never tamed. It left its den towards night, and re-entered it towards 
dawn. It also came out at certain hours during the day for its food, and also 
to drink water, of which it was extremely fond. But even by the side of the 
food a little cage full of moss was always placed, and if the dish was not 
exactly to its liking, it retired until something more to its taste was provided. 
Sometimes it would carry a portion of it into the cage, a fact which M. Guyon 
thinks enough to prove that this Lemming, like the others of its genus, lays 
up provisions for the winter, contrary to the general opinion of travellers. 
When awake, it was perpetually gnawing at the doors, the wainscoting, and 
even iron bars. If interrupted in this occupation it would utter loud cries 
and pour forth a copious saliva on anyone who tried to seize it. It mani- 
fested a disposition to attack a bird which flew about the apartment in which 
it was kept, but perhaps this was the effect of curiosity alone. It exhibited 
sometimes a certain amount of sociability. If its own habitual cry of cui-cui 
was imitated, it would come forth, but seldom advanced towards the person 
who spoke, and then it remained at some distance ; nor did it ever approach 
the fire throughout the winter. — Comptes Rendus, August 20th. • 
