22 Transactions of the Society. 



by combining silver and cadmium in suitable proportions, electrode^ 

 are produced that have a very high melting-point, and that under 

 ordinary conditions will produce a very good arc. The proportion 

 of silver to cadmium may conveniently be 00 p.c. cadmium to 40 p.c. 

 silver, and such an alloy has a melting-point above 700° C. 



On examining the spectrum given, it is found that the principal 

 lines are situated in the blue, green, and red portions of the spectrum, 

 so that it is possible to obtain a monichromatic source of light at 

 will in either of these regions, and of almost unlimited intensity— 

 the intensity, in fact, being only dependent on the size of the 

 electrodes, and the number of amperes used. If an attempt is made 

 to run an arc with pure-metal electrodes of almost any sort, it will 

 be appreciated how very difficult it is to so adjust the arc that any- 

 thing approaching a constant source of light may be obtained. With 

 the cadmium-silver arc the conditions are very much better, but to 

 obtain the best result the electrodes should be rotated in opposite 

 directions during the time that the arc is burning. This method 

 is, of course, one perfectly well known, and requires no further 

 explanation here. It is not one that entails any considerable com- 

 plication in apparatus, in fact, it is quite practicable for the short 

 exposures required in photo-micrography, to have a hand-driven 

 type of mechanism, which is both inexpensive and sufficiently 

 efficient. 



It may be urged that owing to the fact that the electrodes 

 suggested are an alloy, that the spectrum of silver will be equally 

 evident with that of cadmium, and this is, of course, the case ; but 

 fortunately the spectrum of silver is such that it does not interfere 

 to any extent with the cadmium spectrum, the principal cadmium 

 lines, in fact, being so brilliant that the others are almost negligible. 

 The red cadmium line, with a wave-length of 6438 -10 m., is a 

 particularly brilliant one not far from the region of the orange, so 

 that it has considerable visual luminosity. The green line wave- 

 length, 5085 10 m., is in the most brilliant portion of the spectrum, 

 and running the cadmium silver arc with 10 amperes, a fully 

 •exposed plate may be obtained in a few seconds, even with the 

 highest powers, using, of course, a suitably sensitized plate. The 

 blue line wave-length, 4799~ 10 m., lias very high luminosity for the 

 blue part of the spectrum, so that it may be used for work with 

 diatoms, or where the greatest visual resolution is required. It 

 might be interesting to note that the bright lines in the cadmium 

 spectrum are extremely narrow ones. It follows that the light 

 transmitted, whether screened off, or obtained by spectroscopic 

 methods, is of one wave-length. 



In the case where this arrangement is used in conjunction with 

 a monochromatic light apparatus, in which the light is split up by 

 the aid of prisms, the slit may be opened very wide indeed so 

 that the light transmitted becomes a very broad band, quite 



