292 The Magnesium and Electric Light [";::;,Vnl\"urT,Tm 



trie liglit from being generally adopted as a source of illumination 

 in the preparation of photographs of the size of the object, or smaller, 

 proved of immense advantage in the reproduction of the feeble 

 microscopical images of highly-magnified objects, and that the 

 pictures were hence clearer and better defined than any photographs 

 of similar objects I had hitherto seen produced by sunlight. I found 

 also that the Electric light was so much more manageable than 

 sunlight as a source of microscopic illumination, that I could readily 

 arrange it to produce negatives with much shorter exposures than 

 are indispensable with the sun. 



The Magnesium light shared these qualities to a high degree, 

 but I found that its best work was done when the object was not to 

 be magnified more than a thousand diameters, and that there were 

 certain limitations to its use on test-objects which will be referred to 

 in the sequel. 



With one or the other of these artificial lights as a source of 

 illumination, I have prepared a considerable number of negatives 

 of interesting microscopical objects, of which a few are appended to 

 this report by way of illustration, while the others will be laid before 

 you in future reports on special subjects. 



The Magnesium and Electric lights are mentioned as possible 

 sources of illumination for the production of Photo-micrographs by 

 Dr. Lionel Beale, in the foui'th edition of his ' How to Work with 

 the Microscope,' page 275. I am not aware, however, that anyone 

 has made successful negatives with high powers with either of these 

 lights prior to the experiments here recorded. There are in the 

 Museum a few photographs with low powers taken with the Mag- 

 nesium hght by Dr. C. F. Crehore, of Boston, Mass., who kindly 

 presented them August 3, 1866. Negative No. 90, old Micro- 

 scopical Series, Army Medical Museum, represents a few villi from 

 the small intestine of a mouse, photographed by the Electric light 

 with a Toth objective of Wales arranged to magnify 84 diameters. 

 The Electric light was produced by forty Bunsen's cells, and as I 

 had no Electric lamp at the time, I held the carbon points in two 

 retort holders, and managed as best I could, during the exposure, 

 the uncertain light thus produced. I know of no other Photo- 

 micrographs than the above to have been actually made by the 

 Electric or the Magnesium hghts ; certainly if any have been, they 

 have not been sufficiently successful for theii' authors to be willing 

 to give them any degree of pubhcity. I have no hesitation, there- 

 fore, in claiming for the Museum and for myself the credit of having 

 demonstrated the serviceable character of these lights as sources of 

 illumination for the preparation of negatives with high powers, and 

 of having devised a simple method which brings their use within the 

 reach of every microscopist. 



I propose now to sketch briefly the process by which negatives 



