New Method of Using the Electric Arc. By E. B. Stringer. 277 



In order that the arc may be perfectly steady the carbons 

 should be small ; solid ones of 6 mm. diameter are the best to use, 

 and the purer the better : the purest I have yet tried are those 

 known as Noris carbons. The arc must, of course, be a compara- 

 tively long one, about \ in., to which end the voltage should be 

 high, not less than 100 ; and a hand-feed lamp such as Daven- 

 port's will be found much the best. It is also absolutely necessary 

 to protect the arc from air currents by enclosing it in a lantern 

 body, and the current used should not be less than 6 amperes. 



It may be added that the method has also the advantage of 

 completely doing away with the excessive light and heat which 

 proceed from the positive carbon. The light is very apt to cause 

 flare in spite of every precaution, and the heat is liable to damage 

 balsam and styrax mounts even through the G in. of water which 

 the trough contains. With the arc alone there is danger of neither 

 of these things. 



The two lantern slides are of Plcvrosigma angulahim dry, and 

 Coscinodiscus astcromjjh edits in styrax, taken by the above method 

 with Zeiss' 3 mm. oil apochromatic objective 1ST. A. 1*4 and 8- 

 compensating eye-piece, the full aplanatic cone from Powell's dry 

 apochromatic condenser, and a camera length of 32 in. ; giving 

 a magnification of 2200 diameters. The exposures were 15 and 

 20 seconds respectively, the current used by the lamp being 

 6 amperes. 



The arc light is, of course, known to be very rich in ultra- 

 violet rays. But, if an image of the arc, formed by condensers of 

 pure white glass, be received upon a fluorescent screen (one of 

 those used in Eontgen ray work, of barium platinocyanide, is the 

 most convenient) it will be seen that the ultra-violet rays proceed 

 to a very large extent, if not entirely, from the arc itself. The 

 arc is seen to consist of a central core, surrounded by a kind of 

 lambent flame ; and it is from this central core that the ultra- 

 violet rays proceed, as is evident from the powerful excitation of 

 the fluorescent substance. The central core evidently consists of 

 incandescent carbon vapour, whilst the surrounding flame is due 

 to the combustion of the carbons in the air. 



If the spectrum of the arc be projected by means of a prism 

 of good white glass upon the fluorescent screen, it will be seen to 

 lie very much prolonged into the ultra-violet region, principally in 

 two broad bands which appear a bright green. Even on a screen 

 of ordinary card these bands may be seen in a carefully darkened 

 room, as the pale lavender grey described by Herschel; and when 

 a trough containing a solution of quinine is held in front of the 

 slit, they ore completely cut off. 



But it appears quite possible that these ultra-violet rays, which 

 we are at present obliged to cut off because our lenses are not 

 corrected for them, may one day be brought into use, and may 



