16 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The carbon arc was first attacked and it proved to be a difficult 

 subject for investigation. The particular lamp used was of the en- 

 closed type, having the globe fitted with a short side tube and a quartz 

 window so as to keep the arc as steady as possible without losing the 

 ultra violet. To the same end it was found desirable to adjust a 

 magnet behind the arc so as to keep it burning on the side of the 

 carbons next the thermopile instead of wandering round and round 

 the carbons in the usual manner. 



The arc thus operated gave a prodigious amount of ultra violet 

 radiation, showing a continuous spectrum far down into the ultra 

 violet and the three enormously intensive carbon bands usually 

 ascribed to cyanogen, one of them in the extreme violet and the other 

 two near wave lengths 380 n/x and 360 /x/x respectively. Reduced to 

 the standard distance the deflection due to the ultra violet cut off by 

 the Euphos glass amounted to 74 cm., being 30 % of the whole energy 

 which passed through the quartz cell. It has, of coiu'se, been long 

 known that the naked electric arc gives off very powerful ultra violet 

 radiations and its effect in the production of ophthalmia electrica 

 has been known for more than half a century, but in this case the 

 extent of the ultra violet activity was somewhat unexpected. 



It was undoubtedly considerably enhanced by the intensive cyano- 

 gen bands as regards that portion of the radiation lying near the visible 

 spectrum, but on the other hand the extreme ultra violet, wave length 

 300 iJLiJ. and less, is unquestionably stronger in the case of an open arc 

 than in the enclosed arc on account of the very intense continuous 

 spectrum emitted from the crater, which is much lessened when the 

 arc is enclosed. No separation between these parts of the ultra 

 violet was attempted with the lamp under consideration since its 

 unsteadiness was a constant source of annoyance and the ordinary 

 variations of independent readings from the mean amounted to o 

 or 6%. It was sufficiently evident, however, that a powerful en- 

 closed arc in a globe which permits all the radiations to pass is an 

 enormously powerful source of ultra violet light. The carbon arc, 

 however, is rapidly passing out of general use so that attention was 

 next directed to the luminous arc. 



Magnetite Arcs. — The magnetite arc is one of the commonest and 

 most generally useful outdoor illuminants. It gives a very intense 

 nearly white light due almost wholly to the arc stream itself. The 

 spectrum of this, the active electrode being composed almost wholly 

 of the oxides of iron and titanium, is immensely complicated, contain- 

 ing thousands of bright lines so closely packed as almost to obtain 



