SCIENTIFIC SUMMARY. 
331 
the shaded bands of a spectrum of the first order into the narrow ones of 
one of the second order, it is in consequence of an increase of temperature. 
9. The sparks of different induction-coils present spectral differences to the 
inequality of the temperatures developed. 
Is a Circular or an Ordinary Compass the Best ? — This is an important 
point, and it is decided in favour of the round one by M. E. Duchemin, who 
says that it has the following advantages over the ordinary form (see 
“ Comptes Rendus,” vol. lxxvii. p. 890) : — 1st. A magnetic power, for a 
given diameter, double that of a needle whose length is equal to this dia- 
meter. 2nd. The existence of two neutral points instead of one, which has 
the effect of maintaining the position of the two poles constant j the mag- 
netism seems to be so energetically preserved, that even the strongest sparks 
of a Holtz machine do not cause any displacement of the poles of the 
magnet. 3rd. A more satisfactory means of suspending the magnet when 
it is well mounted and balanced by a plate of agate ; it seems then to move 
as if placed in a liquid. 4th. An increase in sensibility of the magnet 
proportional to its diameter. 5th. The possibility of neutralizing the 
magnetism of the vessel by means of a second magnetic circle, changing the 
position by an amount calculated beforehand, and thus permitting the com- 
pensation of the compass before the sailing of the vessel. This idea was 
suggested by Captain D. Venie. 
A Vitroscope turned into a Tonometer . — M. Terquem, says the 11 Chemi- 
cal News,” uses graduated tuning-forks with slides and a small lens 
(Lissajous). By moving the slides the rates of vibration can be varied 
throughout an octave. He has other tuning-forks with lenses, but un- 
graduated. One of the former (or standard forks) and one of the latter 
(or auxiliary forks) are fixed at right angles in the same horizontal plane, and 
may thus make horizontal and vertical vibrations respectively. Some 
powdered antimony is gummed to a branch of the auxiliary fork, and this, 
illuminated with a lamp, gives bright luminous points. The forks are first 
brought into unison, and then the variations of the elliptical curve (got 
from co-existence of the vibrations) when the slides are moved are noted, 
along with the number of beats in a given time. By a few further steps 
one is enabled to ascertain the absolute number of vibrations for each note. 
Influence of the Magnet over Polarisation . — Professor Villari has a 
contribution api'opos of this question in “ Poggendorff’s Annalen,” which 
is thus briefly abstracted in “ Silliman’s American Journal.” A beam 
of polarised light was passed through the poles of a powerful electro- 
magnet, and a glass cylinder interposed, which acted like a cylindrical 
lens. This cylinder could then be turned end over end with any desired 
velocity. When not magnetised, it of course produced no effect on the 
plane of polarisation, whether in motion or not. But when caused to 
revolve rapidly, the angle through which it turned the plane of polarization 
was considerably diminished, since in each revolution it remained in the 
axial direction too short a time to acquire its full magnetic polarity. To 
impart sufficient diamagnetic intensity tc be" perceptible by the change of 
plane, at least -0012" was required ; while, to produce the complete effect, 
0024" was necessary. 
How Physically to detect Thallium easily. — Mr. Crookes, in one of 
