42 MEMORANDA. 
off the lower one, B. Should the thin upper glass be broken, 
it can be instantly replaced, as no cement is required. It is 
merely needful to remove the fragments and slip a fresh glass 
in. We do not know any compressorium that is at once so 
accurate and so easily used. It often happens that, on ac- 
count of the trouble of an ordinary compressorium, a micro- 
scopist simply uses a slide and a piece of covering glass, and 
finds, when too late, that an exact means of regulating the 
pressure would have been desirable. With Mr. Ross’s new 
pattern the convenience is so great that it should always be 
employed if there is a chance of the screw motion being ad- 
vantageous.—F rom the Intellectual Observer for Oct., 1863. 
Cheap Lantern Polariscope.— Mr. Samuel Highley, the 
microscope and philosophical instrument maker of Green 
Street, Leicester Square, has just introduced an arrangement 
that has long been a desideratum with those who delight in 
popularising science, namely, a polariscope that could be 
used in conjunction with the numberless magic-lanterns that 
are now scattered over the kingdom and our colonies, with- 
out entailing the risk and trouble of sending them to the 
optician “ to be fitted ” with such an adjunct, and at a cost 
that is within the means of most persons who indulge in 
such pursuits. 
Doubtless most of our readers are familiar with the mag- 
nificent chromatic phenomena of polarized light, too seldom 
shown in our lecture-rooms, on account of the hitherto costly 
character of the apparatus necessary for its proper display, 
though frequently shown on a small scale in microscopes at — 
soirées and on our drawing-room tables. All who have seen 
such instruments will readily understand how beautiful the 
effect must be when such objects are projected on a screen 
by means of a powerful light. 
By the aid of the polariscope we are enabled to make slices of 
crystals, homogeneous in aspect, reveal their “inner nature,” 
as the Germans have it; so that by the characteristic appear- - 
ance of the rings produced, or angle between the optic axes, 
we are enabled to determine between the two species of a 
mineral which may be identical as to chemical constitution. 
Thus, if two fragments of crystal came into our hands, by a 
chemical assay we migut find that each consisted of carbonate 
of lime; but an optical examination in the polariscope would 
at once show us that one piece was the species calcite, while 
the other was arragonite; the former, belonging to the 
hexagonal system of crystallization, being characterised by 
. yo re oh 
