276 Transactions of the Society. 



V. — A New Method of Using the Electric Arc in Photomicrography* 

 By E. B. Strixgee, B.A. F.E.M.S. 



(Bead April 17th, 1903.) 



The method I have adopted consists, briefly, in employing the 

 radiation from the electric arc itself, altogether separated from 

 that of the incandescent carbons. This, modified by certain light- 

 filters, yields a powerful violet monochromatic light, on the 

 extreme limit of visibility. 



If an image of the arc formed by condensers of good white 

 glass be thrown upon an opening in a blackened screen, the open- 

 ing being of such a size as to allow only the radiation from the arc 

 to pass, the separation is easily effected. The light thus obtained 

 is of a warm violet colour, and very rich in ultra-violet rays, as 

 is shown by its great power of exciting fluorescence in such bodies- 

 as solutions of quinine and resculin and in the platinocyanides. 

 Its spectrum, examined with an instrument of fairly high dis- 

 persive power, is a fluted one, especially remarkable for a bright 

 group of lines in the blue, and a band, still brighter, in the extreme 

 violet, which is separated from the rest of the spectrum by a dark 

 interval of some length. If now a solution of ammonia-sulphate 

 of copper be interposed of such a strength as only to transmit the 

 violet band, and if the ultra-violet rays be cut off by another 

 trough containing a solution of sulphate of quinine, we obtain the 

 light I have described. 



The six lines of which the violet band is made up are close 

 together and differ only very slightly in wave-length, so that the 

 light may be considered strictly monochromatic ; and though 

 visually of little intensity its actinic power is great, and at 2000 

 diameters the necessary exposure is only 15 seconds. There is,, 

 moreover, light enough to focus by with ease at this magnification. 



The apparatus which I described in the Society's Journal of 

 April 1898 proves to be well adapted for the work. The con- 

 densers are aplanatic, and of the purest white Jena glass, and 

 throw a sharp magnified image of the arc upon the substage dia- 

 phragm : and as this image is formed by a pencil of slightly 

 divergent rays, the working of the substage condenser is in no 

 way interfered with. 



A shutter having pneumatic release is attached to the camera 

 front. Then, with one hand on the feeding adjustment of the arc 

 lamp and the other holding the pneumatic ball, the image of the 

 arc is easily watched upon the substage diaphragm, and the ex- 

 posure made at the right moment. 



