274 Progress in Science. (April, 
otherwise produced, is covered with a few sheets of copying paper wetted with 
an acid solution of prussiate of potash, and then screwed into the press. The 
characters or design upon the varnished plate are formed of bare metal, and in 
these parts an electric current is set up ; this action permits of the union of the 
iron with the potash, and the consequence is, Prussian blue is formed in lines 
corresponding to those upon the varnished plate. Copies thus produced in 
blue ink may be printed at the rate of 100 per hour. The patent is the property 
of Messrs. Waterlow and Sons. 
Ata recent meeting of the Royal Society, a paper was read by Professor J. 
Clerk Maxwell, F.R.S., ‘ On the Induétion of Electric Currents in an Infinite 
Plane Conduéting Sheet.” The paper contained a scientific demonstration of 
what takes place in a conductor of infinite extent, acted upon by a system of 
magnets or electro-magnets. It is of singular importance in giving an insight 
into the action of dampers on magnetic needles, such as Harris’s copper ring 
surrounding a ship’s compass ; in fad, into those causes which check the vibra- 
tion of needles, and which seem to exercise a kind of unseen friction on sus- 
pended magnets. The author states that former writers do not take into 
account the induétive action of the currents on each other, though the existence 
of such an action was recognised. In his investigation a system of magnets 
or ele@ro-magnets is supposed to exist on the positive side of the sheet; and 
to vary in every way by changing its position or its intensity. The nature of 
the currents induced in the sheet, and their magnetic effect at any point, andin 
particular their reaction on the electro-magnetic system which gave rise to 
them, are determined. The result is presented in a simple form by the aid of 
the principle of images which was first applied to problems in electricity and 
hydrokinetics by Sir W. Thomson. The essential part of this principle is that 
we conceive the state of things on the positive side of a certain closed or 
infinite surface (which is really caused by actions having their seat on the 
surface) to be due to an imaginary system on the negative side of the surface, 
which, if it existed, and if the action on the surface were abolished, would give 
rise to the actual state of things in the space on the positive side of the surface. 
The state of things on the positive side of the surface is expressed by a 
mathematical function which is different in form from that which expresses the 
state of things on the negative side, but which is identical with that which 
would be due to the existence, on the negative side, of a certain system which 
is called the image. The image, therefore, is what we should arrive at by 
producing (stretching), as it were, the mathematical function, as far as it will 
go; just as in optics the virtual image is found by producing the rays, in 
straight lines, backward from the place where their direction has been altered 
by reflection or refraction. In the case of a plane conducting-sheet there is a 
moving train of images. If positive and negative images, according to the 
sides of the plane, are formed and started at given intervals of time, a train 
or trail will begin with a single positive image, followed by an endless succes- 
sion of pairs of images. ‘This trail when once formed continues unchangeable 
in form and intensity, and moves as a whole away from the conducting-sheet 
with a constant velocity. If the conduétivity of the sheet were infinite, or its 
resistance zero, that velocity would be zero. The images, once formed, would 
remain stationary, and all except the last formed positive image would be 
neutralised. Hence the trail would be reduced toa single positive image, and 
the sheet would exert a repulsive force on the pole, whether the pole be in 
motion or at rest. This case does not occur in nature as we know it. Some- 
thing of the kind is supposed to exist in the interior of molecules in Weber’s 
theory of diamagnetism. 
With the first day of this year the eleGtric light was introduced at the South 
Foreland Lighthouse, which is situated between Dover and Deal. The posi- 
tion is of great value owing to its situation with regard to the opposite coast of 
France and the approaches to the’ North Sea and the mouthof the Thames. The 
completion of the works at the Foreland will form a triangle of electric lights 
described by those of Dungeness, Cape Griznez, and the South Foreland. The 
South Foreland towers are 449 yards apart, and their lights are respectively 
372 feet and 275 feet above high water of spring tides. Buildings for the 
