Fig. 2.—THE HERSCHEL-BROWNING DIRECT VISION SPECTROSCOPE.
Fig. 3.
A, another form of gas lamp; B, the flame; C, adjusting stand to carry the platinum wire and the lamp.
HOW TO USE THE SPECTROSCOPE.
Screw the tube carrying the knife edges at the small end into the prism bore, and the telescope into its proper ring. Now place any common bright light exactly in front of the knife edges, and while looking through the telescope arrange the knife edges until a bright and continuous spectrum is visible.
TO OBTAIN THE BRIGHT LINES IN THE SPECTRUM GIVEN BY ANY SUBSTANCE.
Remove the bright flame from the front of the knife edges, and substitute in its place the flame of a common spirit lamp or, still better, a gas-jet known as a Bunsen’s Burner. Take a piece of platinum wire about the substance of a fine sewing needle; bend the end into a small loop about the eighth of an inch in diameter; fuse a small bead of the substance, or salt, to be experimented on into the loop of the platinum wire, and attaching it to any sort of light stand or support, bring the bead into the front edge of the flame, a little below the level of the knife edges. If the flame be opposite the knife edges, on looking through the eye-piece of the telescope the fixed lines due to the substance will be plainly visible. When minute quantities have to be examined, the substance should be dissolved, and a drop of the solution, instead of a solid bead, be used on the platinum wire.
The delicacy of this method of analysis is very great. Swan found, in 1857 (Ed. Phil. Trans., vol. xxi. p. 411), that the lines of sodium are visible when a quantity of solution is employed which does not contain more than 1⁄2,500,000 of a grain of sodium.