[Read before the Philosophical Institute of Canterbury, 1st October, 1874.]

It is a matter of some difficulty, when using an ordinary electric lamp, to project the spectra of several metals on the screen in quick succession. A special piece of apparatus for this purpose is described by Mr. Lockyer in his book, in which a circle of ordinary gas carbon cups are used. I have made the following modification of the ordinary lamp, and have found the result very successful.

The lower carbon is taken out, and into the holder is placed a stout cylinder of brass with a pin turned on the top; on this pin is a plate of gas carbon three inches in diameter and half-an-inch thick, with a hole in the centre through which passes the brass pin. The carbon disc is thus free to revolve round the top of the pin; near the edge is a circle of small hollows into which the pieces of the metals are placed. The upper carbon would now come in contact with the top of the brass pin; in order to bring the upper carbon above the hollows containing the metals, the top arm is provided with a slot to slide out. With this arrangement the spectra of a number of metals may be projected on the screen in a few minutes with the greatest ease, the metals burning with an amount of steadiness I have not seen before. A piece of copper about the size of half a pea was placed in one of the hollows and completely burnt away without any attention.

The lamp I have is one of Browning's, and it acts most satisfactorily, but the positive carbon is placed at the top. I find that in order to burn the metals with a steady light the positive must be at the bottom; when it is at the top it invariably begins to vibrate after a little time, and sometimes throws the metal out of the cup. I have not yet noticed if this is the case when using the ordinary carbon cups. The steadiness of the arc is probably due to the fact that the large mass of the carbon plate prevents it from becoming much heated, so that the convection currents, which I believe to be the cause of the arc (see my paper, Phil. Mag., Dec., 1873), have naturally a greater tendency to spring from the heated metal than the cold carbon. So small is the heat developed in the carbon that it remains quite black, and may be safely turned round with the hand.