484 
POPULAR SCIENCE REVIEW. 
constant electric current is formed : (1) The deposit of silver is made not in 
the capillary slit, hut on the iron ; (2) When the wire is removed the 
deposit is formed in the slit and on the edges along the side of the split hell- 
glass. The capillary action is as powerful as an electrical action. M. 
Becquerel continues to improve his experiments ; for the split hell-glass he 
substitutes prisms of crystal glass pierced with a small hole ; the slit or 
fissure is replaced by plates of glass with edges in contact, or even by sand ; 
and he has thus obtained effects of silvering, gilding, and platinising. — Vide 
Comptes Rendus , July 1, and Chemical News . 
A new Polarising Photometer was described by its inventor, Mr. W. 
Crookes, F.B.S., at the British Association meeting at Dundee. He was 
unable to exhibit the instrument itself, but he gave the following description 
of it : Two discs, emitting natural — not polarised — light, are placed in front, 
and at a considerable distance behind is a doubly-refracting prism of 
Iceland spar, rendered achromatic by a piece of glass, which will separate 
the light emitted by the two discs into three ; but, for the purposes of the 
instrument, the outer two must be disregarded. The difference of intensity 
between the original self-emitting discs is proportioned to the free polarised 
light found in the central disc of light. This is again split up, and the 
difference of intensity between the first discs is ascertained from the difference 
between these final ones. In comparing the light of two stars, Mr. Crookes 
makes use of Arago’s Polarimeter, which, twisted in one direction, gradually 
cuts off one kind of light, and when twisted in the opposite direction, cuts 
off the other kind of light, so that the intensity of the light is measured by 
the angle through which the instrument must be turned. 
Recent Memoir on Optics . — Herr Dove, the celebrated physicist and me- 
teorologist, has presented to the Academy of Berlin a memoir in which he 
discusses the following important points : — 1. The formation of white by 
uniting the colours of the spectrum; 2. The subjective colours in the 
electric spark ; 3. The inversions in monocular or binocular vision of per- 
spective drawings or transparent bodies; and 4. The polarization of light 
by successive reflections. — PInstitut, August. 
Improvements in Voltaic Piles . — Certain improvements have been sug- 
gested by M. Zaliwski Mikorski. The latest experiments of the physicist 
show that by increasing the height of the elements without altering their 
base, a current proportionate to the height may be obtained. He recommends 
the following method for increasing the energy and permanency of a Bunsen’s 
battery Place two porous vessels one within the other : into the first, con- 
taining the carbon, pour nitric acid ; into the second, sulphuric acid ; finally, 
into the outer vessel, containing the zinc, pour sal-ammoniac. There is no 
effervescence, and the zinc undergoes no useless destruction. 
An excellent Popular Illustration . — Sir W. Thomson, in describing a small 
electrical machine to the British Association, employed the following happy 
analogy to convey an idea of the purpose and mode of action of the new in- 
strument : — The principle of the machine is that of the u Successful Mer- 
chant ” who commenced life with a capital of \d., and, after a month’s per- 
severing industry, realised the handsome sum of 1/., and continued to go on 
increasing his capital at a compound rate of interest. The object of the in- 
strument referred to is not indeed to increase money but electricity, and 
