TRANSACTIONS OF THE SECTIONS. 19 
examining absorption spectra was illustrated by an extreme case—permanganate 
of potassium ; ordinary drawings of the bands were made for four different thick- 
nesses, and the same were expressed in Mr. Sorby’s ingenious notationof numerals 
and dashes and dots, which four figures bore little or no apparent relation to one 
another, but they were explained by, and included in, the figure obtained when the 
hollow wedge was employed. Figures were also exhibited showing the absorption 
of light by cruorine, hzematine, and cineal in alum, which illustrated in various 
ways the importance of observing the effect of the varying thicknesses of these 
substances. 
The author suggested the use of the hollow wedge as far back as the Cheltenham 
Meeting in 1856, and exhibited there, and published afterwards, many diagrams of 
absorption spectra thus obtained. The reason why other observers have made 
little use of it was believed to be twofold—it is too simple to please some, requi- 
ring no apparatus beyond a common window-shutter, the wedge, and a good prism, 
while it demands a little more thought in adjustment than the test-tube does, 
seeing it also bends the ray of light. It may be advantageously placed in front of 
spectroscopes of the ordinary form; and as the prismatic analysis of transmitted 
light is becoming a matter of great importance, it seems desirable to adopt the best 
methods. 
Sur une action particuliere de la lumiére sur les sels @argent. 
By Professor Morren. 
Enectrriciry, Magners. 
On a further development of the Dynamo-Magneto- Electric Machine. 
By W. Lavp, F.RAS. 
At the Meeting last year the author brought before the Section one of his small 
dynamo-magneto machines, the first that had been made upon that principle. 
The author has since constructed a much larger machine, and it may be interesting 
now to give some particulars respecting it. The object in constructing it was to 
supply a good electric light for the purpose of lecture demonstrations. It is con- 
structed upon the double armature principle, both armatures being placed end to 
end, so that their magnetic axes cross each other at right angles. The short arma- 
ture contains 108 feet of very stout copper wire, and sends its currents into 240 lbs. 
of copper wire surrounding the electromagnet, exciting a large amount of mag- 
netism in the body of the machine. And as the second armature is also made to 
revolve between the poles of this electromagnet, a sufficient effect is produced at 
the two ends of the 312 feet of very stout copper wire (which is wound upon 
it) to produce a good electric light from the carbon-poles of the regulator. But in 
order to make that light sufficiently continuous, it is requisite that the armatures 
should revolve from 1800 to 2000 revolutions per minute; but as the armatures 
haye to be magnetized and demagnetized twice during each revolution, there would 
be in the latter case 4000 flashes of light per minute. Now it has been shown 
that every time iron becomes magnetized it is elongated, and again shortened 
when demagnetized. At every alteration, therefore, of the condition of the iron 
some small amount of heat must be devolved, and would increase to such an ex- 
tent that, if unchecked, it would in the course of time be so great as to destroy the 
insulation of the wire. 
The author did not wish it to be inferred that the sole cause of heat is due to 
the elongation of the iron. Doubtless the electric currents passing through the 
wire would produce heat; but he believed the quantity produced by that means 
would be small as compared with that produced by the elongation of the iron itself. 
The author gave the following reasons for entertaining tkis opinior. One of these 
magneto-machines driven by steam-power was lately used in connexion with a 
large inductorium, and after a few hours it was found that the copper or primary 
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