Royal Microscopical Society. 153 



solar rays or from the electric lamp, either by means of a prism as 

 recommended by Count Castracane, or still more conveniently by 

 passing the rays through a coloured solution. I use, ordinarily, a 

 solution of the ammonio-sulphate of copper, which gives a bluish- 

 violet light approximating monochromatism sufficiently for all prac- 

 tical demands, whether of observation or photographic representa- 

 tion. When I compare the images of a difficult test as seen with 

 any objective by such monochromatic light, with the best attainable 

 with the lamp-light illumination ordinarily employed, I must confess 

 to a feeling of astonishment that this simple and potent optical re- 

 source is not more generally employed by microscopists, aside from 

 all questions of photography. Not the least of its advantages is the 

 fact that in the case of those objects which decompose light readily, 

 the optical effects produced by this circumstance are at once ehmi- 

 nated, and the observer is consequently better able to judge reasonably 

 with regard to the actual form. 



Now, in the case of transparent objects, I do not hesitate to 

 assert that whatever can be discerned by " the human eye armed 

 with the microscope " if monochromatic illumination is employed, can 

 be copied photographically in a manner that faithfully represents 

 every detail seen. And such representations have this superiority over 

 mere drawings or descriptions, that they are purely objective, and the 

 imagination or enthusiasm of the observer, which plays so consider- 

 able a role in most drawings and descriptions, is wholly excluded. 



I cannot, therefore, admit Dr. Pigott's statement that illumina- 

 tion is a " vexed question for the photographer," and his aUusion to 

 the failures of " the actinic glasses " is apparently penned without a 

 consideration of the fact, that the immersion objectives of high 

 powers, which he so justly praises, requu-e no special corrections 

 to fit them for photographic use, and are the very ones actually 

 employed. As for the Amphipleura pellucida, there is nothing to 

 prevent anyone from photographing it quite as well as he may be 

 able to see it ; to this, however, I shall recur again. 



I sincerely beheve that if microscopists generally would take the 

 trouble to master the simple steps necessary to enable them to record 

 their actual observations by photography, many bitter and unneces- 

 sary disputes would be avoided, and the sohd progress of inquiry 

 into minute structure would be materially advanced. I am not, 

 however, disposed to exaggerate the just importance to be attached 

 to photographic representations, and must therefore add a few words 

 with regard to the real hmitations of their usefulness. 



All that can properly be claimed for photo-micrographs is, that 

 if the chemical work is well done, the picture will be a faithful 

 representation of whatever the observer is able to discern with 

 monochromatic light. Success in the chemical process is best and 

 most economically secured by employing a professional dark-room 



