THE SPECTRUM 317 



magnesium may be employed. White glass lenses are much 

 superior for these experiments, as with the following. 



181. Fluorescence. Many experiments in Fluorescence, 

 of great beauty, are easily made with the lantern. For those 

 which depend chiefly on the conversion of the invisible rays 

 at the violet end of the spectrum into blue light, it is advisable 

 to have a radiant of high actinic power, such as the arc, or 

 burning magnesium ribbon ; and even then the effect is 

 heightened if a quartz condensing lens and prisms can be 

 employed, as crown glass, and still more carbon disulphide, 

 strongly absorb these rays. Lenses and prisms of fine ' white ' 

 flint, however, also give very good results. A temporary 

 magnesium light can be got with little trouble by passing 

 three ribbons through a brass tube, an assistant watching the 

 combustion through a darkened glass, and feeding the ribbons 

 accordingly by hand. The end of the tube should be in the 

 focus of a large condensing lens, and the whole be fitted up 

 separately, since magnesia makes a great mess in an ordinary 

 lantern. Sulphur burnt in oxygen gives a strong actinic 

 light, and even in air is sufficient for some effects. 



With any such light, if a sheet of paper or card, washed 

 repeatedly with a saturated solution of quinine sulphate in 

 water acidulated with sulphuric acid, is pinned on the screen 

 so as to cover and extend beyond the visible spectrum thrown 

 through even a glass prism, a visible extension of the violet 

 light will be produced, especially if the brighter part of the 

 spectrum be shaded off to prevent the feeble light being over- 

 powered. A glass tank some inches long, with flat ends, 

 filled with the same solution, will mark the cone of light 

 passing through it from a large lens by a beautiful sky-blue 

 fluorescence. Solution of assculm, or a decoction of horse- 

 chestnut bark, also gives a brilliant blue. 



The reciprocity of absorption and fluorescent power in the 

 beam is shown by first showing the extension of the visible 

 spectrum as before, and then interposing the tank of quinine ; 



