ToSZ^l. junerisTo"'] as applied to Photo-micrography. 291 



last I began to conduct in person a series of experiments, intended 

 to devise means for escaping certain difficulties which had hitherto 

 prevented the successful preparation of Photo-micrographs of speci- 

 mens selected from the valuable and daily-increasing series of per- 

 manently-mounted microscopic sections of normal and pathological 

 tissues, which form so interesting a portion of the treasures of the 

 Museum. In these experiments I used the sun as a source of illu- 

 mination, and following the process which I have described in full 

 elsewhere,* I had no difficulty in arraoging a method by the aid of 

 which this class of objects could be photographed quite as success- 

 fully and readily as the diatoms and other test-objects which had 

 previously been so satisfactorily reproduced in this section of the 

 Museum. I shall take occasion in the course of a few days to lay 

 before you prints of some of the tissue-preparations thus reproduced. 

 At present it is my desne to call your attention to certain important 

 observations which I had the good fortune to make while my expe- 

 riments were in progress, and which it appears to me cannot fail to 

 be of interest and service to all microscopists. 



During the last week of October and the first two weeks of 

 November, I relied wholly on the sun as the source of illumination 

 for producing negatives. In this period, during which I had but 

 two perfectly cloudless working days, and several fractional days, on 

 which my work was continually interrupted by passing clouds, I had 

 ample opportunity to convince myself that the uncertainty of the 

 weather was a most serious hindrance to the preparation of success- 

 ful photographs of microscopic objects, and I ceased to wonder that 

 European microscopists, who are exposed to a climate even more 

 variable than our own, have not yet succeeded in placing the art of 

 Photo-micrography upon such a basis as to make it a convenient and 

 habitual auxihary in aU microscopical investigations. This desu'able 

 end I believe I have attained ; but it has been by resorting to artifi- 

 cial lights, and thus making the success of the process whoUy in- 

 dependent of the weather. 



On the 12th of November I commenced a series of experiments 

 with artificial lights, which were most fortunately crowned with 

 success, both the Magnesium and the Electric lights proving ade- 

 quate sources of illumination for the production of Photo-micrographs 

 even with the highest powers. 



For the production of Electric light I used a Duboscq'?5 lamp, 

 set in motion by a battery of fifty small Grove's elements. I found 

 that, with this source of light, photographs could be successfully 

 taken with any power with which pictures can be taken by sun- 

 light ; and I was delighted to find, as I had anticipated, that the 

 very exaggeration of hght and shadow which has prevented the Elec- 



* Circular No. 6, War Department, Surgeon-General's OflQce, Nov. 1, 1865, 

 page lis et seq., ' American Journal of Science and Arts,' vol. xlii., Sept., 1866. 



U 2 



