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. 



