Use of a Metallic Electric Arc in Photomicrography. 23 



sufficient to fill the field with moderate or high-power microscopic 

 objectives. 



It may be urged that the method, although, perhaps, not with- 

 out scientific interest, is impracticable owing to the cost of the 

 cadmium silver electrodes, and it must be admitted that this is 

 not altogether a negligible factor. Cadmium is now, however, not 

 a rare metal, and can be bought cheaply, whilst the value of silver 

 is well known to you all. I may say, moreover, that I have 

 recently obtained some of this alloy from Messrs. Johnson Matthey, 

 the Metallurgists, of Hatton Garden, and that the cost of a single 

 electrode works out at five shillings. This is sufficient for a fairly 

 long run, so that after all the expense would not, in most cases, be 

 an insuperable objection where some special result is aimed at. 



