196 ANNUAL REPORT SMITHSONIAN" INSTITUTION, 19 3 



lamps used for therapeutic purposes may be arc lamps or mercury 

 vapor lamps, but for scientific use the latter type of lamp is usually 

 preferred. 



A common- form of mercury vapor lamp is essentially a specially 

 constructed quartz tube, exhausted of air, and containing liquid mer- 

 cury wliich may be made to separate, and thus an arc may be struck. 

 The wires are connected in such a manner that the spark must jump 

 from the one body of mercury to the other, thus traversing the gap 

 occupied by mercury vapor. 



Quartz is employed for the tube or bulb because of its remarkable 

 transparency to ultra-violet rays, permitting penetration of wave 

 lengths as short as about 1,850 A. Only calcite and fluorspar are 

 known to transmit yet shorter wave lengths. For the lamps designed 

 for therapeutic use, quartz need not be employed, as such lamps are 

 planned to furnish only the ultra-violet rays of wave lengths greater 

 than 2,900 A, these being the rays which come to us in sunshine. 

 Thus a special kind of glass may be made to serve for the tube or 

 bulbs, and the employment of quartz is not necessary for family sun 

 lamps. Lamps embodying bulbs or tubes are known to become less 

 efficient with use. This progressive loss of efficiency is largely due 

 to changes which the radiation effects in the quartz or glass, prob- 

 ably a gradual devitrification, and a discoloration because of the 

 deposition of particles by sublimation. 



FILTERING OUT LIGHT WAVES 



The filters used to cut out the light waves, so that only the ultra- 

 violet may be transmitted, are either plates of quartz on which a 

 thin film of silver has been deposited, or they are glasses deeply col- 

 ored with certain metallic ingredients. 



THE INVISIBLE BECOMES VISIBLE 



When the invisible ultra-violet rays, unaccompanied by light rays, 

 are caused to fall upon certain objects, an astonishing phenomenon 

 may be observed. Objects which in common light have color, may 

 change color; and objects which are commonly colorless, may ex- 

 hibit color — may show red, orange, yellow, green, blue, or violet — 

 any color of the spectrum. Thus, the fingernails, the teeth, and 

 the eyeballs glow with a pale yellow light. A crystal of calcite 

 looks like a coal of fire ; a crystal of fluorite shows a beautiful violet 

 blue. The zinc ore called Willemite, glows brilliantly with a yellow- 

 green light. Indeed, man}^ chemicals, in ultra-violet light, show 

 characteristic color effects. Thus, calomel exhibits a brick-red 

 color, sodium salicylate is violet blue; quinine, violet; petrolatum, 



