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3H OPTICAL PROJECTION lines are produced; then, on shutting it off, the dark band will usually be seen. Such a lamp may also be used between the lime-light and the slit; but the flame should be as far from the slit as convenient, in order to be out of focus. I have found that the brightness of the radiation spectrum may also be increased by condensing the rays from the condensers upon the slit by means of a lens. Absorption may be clearly projected by other methods. Makers of physical apparatus prepare sealed glass tubes about f-inch diameter (which must be of strong and hard combustion- tube), in which are placed some small portions of clean sodium in an exhausted atmosphere of hydrogen (in order to prevent oxidation of the sodium). If this is carefully heated and held in the rays of the spectrum, it will be found to cast a shadow in the orange rays, but not elsewhere. Sometimes it can be so heated as to show dark lines if held over the slit, but this is rather a doubtful experiment. Or it will cast a shadow in the rays from a Bunsen sodium-flame. Mr. Cleminshaw has devised another pretty experiment of this class. Using any apparatus for producing a brilliant sodium-light in the lantern itself, as recently described, it is adjusted so as to give a bright yellow disc on the screen about five feet in diameter by projection through the con- densers. A Bunsen burner is then adjusted in the lantern near to the condensers (protected by a glass) and the orifice about half an inch above the lower edge of the lenses. On holding a carbon rod, or bunch of asbestos fibre, or other medium impregnated with salt, in the flame, a distinct shadow will be cast upon the screen. 1 1 Mr. Sleeman informs me that by an arrangement somewhat similar to this, but using an ordinary incandescent lime to give the spectrum, and using a bunch of asbestos fibre squeezed into the lower end of a glass tube filled with a saturated solution of salt, the brush thus constantly fed being held in the flame in front of the lime, he has obtained sometimes a fair reversal of the D line. I have had no opportunity of trying this method «ince it was suggested to me.