ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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and be of the same size, provided that internal currents are guarded 
against. 
Some sucli effect the author obtained, not, however, by conversion of 
a fused mass into a solid modification, but into a liquid crystallized one. 
To avoid currents in the fused mass, special care was taken to have 
everything perfectly clean, and only very slight thickness was given to 
the liquid layer between stage and cover-glass. In this capillary space 
one of the three substances was heated until the solid crystals were 
converted into the doubly refracting liquid modification. This occurred 
in regular orientation with respect to the solid crystals, i.e. a copy of 
each in its outline was obtained, but with other interference colours and 
different directions of extinction. On heating still further, until the 
doubly refracting liquid was converted into the singly refracting, the 
field of view between crossed nicols became dark by the widening out of 
dark circular spots which formed in the doubly refracting mass. The 
fused mass was then cooled down again, when the doubly refracting 
liquid crystals again appeared with precisely the same outline and 
directions of extinction as before. The author considers that this 
experiment serves to strongly support the idea that non-doubly refracting 
fused masses are regularly crystallized enantiotropic modifications. 
That doubly refracting liquids are so rare as to have hitherto escaped 
discovery, receives some explanation from the fact that with substances 
having many enantiotropic modifications, the crystal system, with 
increased temperature, tends to a higher degree of symmetry, and thus 
finally to the regular system. 
On the History of the Invention of Spectacles, Microscope, and 
Telescope.* — Herr C. Landsberg shows on what uncertain grounds it 
was that the year 1890 was regarded as the 600th anniversary 
of the invention of spectacles, and the 300th of that of the Micro- 
scope. It is impossible either to fix a precise date for these 
inventions, or to give with certainty the names of the inventors. 
The art of cutting and polishing precious stones was known to the 
ancients, and among the relics of this art we posses lenses, both convex 
and concave, which are at least 3000 years old, e. g. the plano-convex 
lens of rock-crystal discovered by Layard in the ruins of Nineveh. It 
can scarcely be doubted that the men who made these lenses were 
acquainted with their magnifying power, and in fact made use of it in 
the execution of those delicate engravings on gems which have been 
handed down to us. An exact description of the effect of spherically-cut 
glass is, however, not to be found in ancient literature ; but Pliny 
mentions that the near-sighted Nero looked at the gladiatorial games 
through a smaragd. The Arabian physician Alhazen (about 1100 
a.d.), who was the first to give an exact anatomical description of the 
eye, showed by his writings that he knew the magnifying effect of a 
segment of a sphere made of a denser material than the air. Later 
writers on optics refer to the observations of Alhazen, but add nothing 
to them. To Roger Bacon (1216-1294), however, much more extensive 
knowledge is ascribed. He is often credited with the invention of eye- 
glasses and the telescope. All that can be gathered from his writings 
Central-Ztg. f. Optik u. Mtchanik, xi. (1890) pp. 2G5, 277. 
