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III. — On the Use of a Metallic Electric Arc in Photomicrograjihy. 



By J. Edwin Barnard. 



(Read November 16, 1910.) 



Some time ago I described to this Society a method of using a 

 mercury vapour arc for visual microscopic work. I explained that 

 it had such advantages that it was probably substantially ahead, as 

 a source of light, of any other illuminant then available — an opinion 

 that subsequent experience has more than confirmed. As the light' 

 is derived from the luminous arc itself, and not from either of the 

 luminous electrodes, it follows that its spectrum is a bright-line 

 one, and that it is possible, by a suitable arrangement of prisms, or 

 by suitable colour screens, to obtain illumination in the Microscope 

 that is truly monochromatic. While for visual work the amount 

 of light so obtained is amply sufficient for use with the highest 

 powers, when it comes to photomicrography the exposures are in 

 many cases prolonged, and sometimes unduly so. I have there- 

 fore been experimenting for some time to see whether a metallic 

 arc with solid metal electrodes might be made available for the 

 purpose mentioned. 



I must at once admit that up till quite recently the results 

 obtained have not been favourable, as it is extremely difficult 

 to get an electric arc with metallic electrodes to burn with any 

 degree of steadiness ; and, further, there is always the difficulty 

 that with most metals the electrodes themselves either quickly 

 melt, or the arc changes its position, owing to bending or displace- 

 ment of the metals. Again, if, for instance, we take an arc formed 

 between iron electrodes, in which the metal itself has not a low 

 melting-point, and which may, by suitable arrangements, be made to 

 run steadily, we are confronted with the difficulty that its spectrum 

 consists of a great number of lines of moderate intensity which are 

 distributed pretty evenly throughout the spectrum, so that, owing 

 to the closeness of the lines one to another, there is no advantage 

 over a source of light with a continuous spectrum, such as an 

 ordinary carbon arc. 



Of all metals giving a suitable spectrum, other than mercury, 

 none so thoroughly fulfil the necessary conditions as cadmium ; but 

 unfortunately cadmium has a particularly low melting-point, rather 

 lower even than that of lead, so that to use it in a pure state is 

 simply impossible. A recent paper by Dr. T. Martin Lowry, on 

 " A New Method of Producing a Cadmium Arc," has suggested that 



