1876.] 
Physics. 
419 
nous observations. Attention should be directed above all to the various 
branches of physics and meteorology, as being of the highest degree of im- 
portance ; then to botany, zoology, and geology ; and, lastly, to geographical 
detail. Should it be possible to establish, in connection with these ArCtic 
stations of synchronous observations, one or more in the Antarctic regions, we 
might expeCt results of inestimable value. 
Several important papers have been read before the Physical Society since 
the publication of the April number of this Journal. At the meeting on 
April 8th Prof. G. Carey Foster, F.R.S., described an instrument which he has 
constructed for illustrating the law of refraction. It is founded on the well- 
known method of determining the direction of the ray after refraction by 
means of two circles described from the point of incidence or centre, the ratio 
of whose radii is the index of refraction. A rod representing the incident 
ray is pivoted at the point of incidence, and projects to a point about 4 inches 
beyond. To this extremity is attached a vertical rod, which slides through a 
nut in another rod also pivoted at the point of incidence. The lower extremity 
of the vertical rod is attached to a link, so fixed as to constrain it to remain 
vertical. By this means the two rods always represent respectively the inci- 
dent and refraCted rays, and the index of refraction can be varied by altering 
the position of the nut through which the vertical rod passes on the rod to 
which it is attached. 
At the same meeting Prof. Foster exhibited a simple arrangement for 
showing the interference of waves, and a method — suggested by Prof. Kundt 
— for showing, in a simple manner, that the air in an organ-pipe is in a con- 
stant state of alternate condensation and rarefaction. 
At the meeting on April 29th the Secretary read a communication from Sir 
John Conroy, Bart., “On a Simple Form of Heliostat.” The author substi- 
tutes two silvered mirrors for the looking-glasses usually employed, and he has 
shown that the loss of light with this arrangement is less than when the light 
is once reflected from a looking-glass. 
Mr. S. P. Thompson described some experiments which he had made on the 
so-called “ Etheric Force.” If the secondary current from an induction coil 
be used instead of a current direCt from the battery the effects are much more 
marked. The secondary current of a Ruhmkorff’s coil is made to traverse a 
short coil of wire, which is thoroughly insulated from the internal core, and 
into the circuit an arrangement is introduced by means of which the spark 
may be made to traverse a variable thickness of air in its course round the 
short coil. It is found that if this spark is very short the spark obtained from 
the internal core is also short; but as we increase the thickness of air to be 
traversed, the spark which may be drawn off increases. The greatest effeCt, 
however, is produced when one terminal of the coil is connected with the 
earth, the spark then obtained being about £ an inch in diameter. Mr. Edison 
considered that the spark was retro-aClive, but Mr. Thompson showed by an 
experiment that deficient insulation might lead to such a conclusion. He then 
proceeded to show that just as the charge given to a gold-leaf electroscope is 
at times positive, and at times negative, without any apparent reason for the 
change, so, if the core of the arrangement employed be connected with a 
Thomson’s galvanometer, the needle will be found to wander irregularly about 
the scale on both sides of the zero. In order to show that these experiments 
are identical with those conducted as originally described by the discoverer, 
the terminals of the induction coil were connected with the coil of an electro- 
magnet, the same means of including a layer of air in the circuit being intro- 
duced. The effeCt in this case was found to be precisely similar to that 
obtained with the special arrangement previously used. With a brush dis- 
charge a Geissler’s tube could be illuminated, and when the layer of air was 
infinitesimal the spark produced was also infinitesimal. It was then shown 
that if the spark at the point of contaCt in the key, when a direCt battery cur- 
rent traverses the coil, be done away with by shunting the extra current which 
gives rise to it, no spark can be obtained from the core. It thus appears that 
po spark is obtained when there is no necessity for an inducing current tQ 
202 
