POPULAR SCIENCE REVIEW. 
rise, but will fall. Mr. Johnson shows a novel instrument for 
registering deep-sea temperatures, where advantage is taken 
of the different degrees of the expansion of metals. Riveting 
two thin pieces of brass and steel together, he lias contrived 
means to register the effects of the small curvature which the 
combined metals must undergo by a change of temperature. By 
this method the pressure of the sea on the bulbs of self-regis- 
tering thermometers is got rid of, which is quite destructive of 
delicacy at immense depths. The same gentleman shows his 
instrument for recording’ deep-sea soundings, in which the fact 
that water is slightly compressible is ingeniously applied. 
MM. Negretti and Zambra exhibit a deep-sea sounding ther- 
mometer on quite a different construction to that of Mr. Johnson. 
In them instrument the bulb of the thermometer is enclosed in 
a second cylinder partly filled with mercury in vacuo ; the pres- 
sure of the water at great sea depths exercises its influence on 
the latter — the internal bulb being quite free from pressure on 
this account. 
Of the ordinary instruments of science — microscopes and 
microscopic preparations — the collection shown in the Trench 
and English departments is excellent. It would be invidious 
to point out any one maker among so many known to fame. 
Single and binocular and museum microscopes are here — from 
the cheapest to the most expensive and complicated forms. 
We may say the same of all the ordinary surveying, nautical, 
and drawing instruments, such as theodohtes, sextants, com- 
passes, and levels. Among the newly invented microscopical 
apparatus, the visitor will perceive, in Ross’s case, the hemi- 
spherical condenser of the Rev. J. Reade, from which, by 
means of double stops and hemispheric lens, one, two, or 
three apertures may be obtained, and the linear markings, no 
matter at what angles, on test-microscopic objects, obtained by 
means even of an ordinary microscope. The application of 
aluminium to small telescopes and other instruments intended 
to be held in the hand, renders them very commodious; a 
sextant here exhibited, only weighs one-half of a similar one 
constructed of the ordinary metal. A contrivance for facili- 
tating the approximate solution of problems in spherical trigo- 
nometry (exhibited by Moore) will be found useful to the 
seaman. The apparatus for spectrum-analysis invented by 
Crookes has now become one of the necessities of the labora- 
tory and the observatory, and replaces the simple prism and 
theodolite originally made use of by Fraunhofer. We trust 
that the inventor may soon set at rest the disputed question of 
the spectra of stars which has within the last few months so 
much engaged the attention of astronomers. 
In some departments and countries the chronometers are 
