1885.] ^^ IFrazer. 



to be repeated under other circumstances to determine this question more 

 certainly. For example, if the camera were focussed sharply on a distant 

 tree, and a negative taken during a violent thunder storm by a lightning 

 flash while the tree is in motion, if the foliage comes out in detail with no 

 perceptible motion shown, the continuance of the illumination would then 

 be proved to be too short a time for its appearance. If, on the contrary, 

 the leaves appear blurred as if moved, then the generally received no- 

 tions concerning the instantaneous character of the lightning flash must 

 be changed. 



Or, if the camera should be focussed on a rapidly moving wheel, and a 

 photographic picture be taken during its illumination by a lightning flash, 

 then the peculiarities of the negative could be utilized, not only to deter- 

 mine the question of the greater or less duration of the flash, but even to 

 measure the actual duration itself. 



It will be observed that the method here suggested substitutes the sen- 

 sitive plate of the photographic camera for the retina of the eye. From 

 the results of Mr, Barker's photographs, it might be inferred that the 

 former is far more sensitive than the latter. If this be the case, then the 

 photographs thus obtained would furnish more precise means for measur- 

 ing the duration of the illumination, and hence of the flash itself, than the 

 method followed by Wheatstone and others. 



The lightning flash contains so large a percentage of the blue rays^of 

 light, that we may fairly suppose that its actinic effects on a photographic 

 plate would be more decided than with equally bright sunlight. This 

 greater sensitiveness of the light of a lightning flash may perhaps account 

 in some degree for the possibility of taking photographic pictures by its 

 means, but it also equally explains the probability of the blurred foliage in 

 Mr. Barker's views being actually due to their movement during the short 

 time they were exposed to the camera, and thus disproves the approxi- 

 mate instaneousness of the flash itself. 



Central High School, 



Phila., Nov. 20, 1SS5. 



Resume of the Work of the International Geological Congress, held at Berlin, 

 Sept. 2S to Oct. 3. 1SS5. By Dr. Persifor Frazer. 



(Read before the American Philosophical Society, November 20, 1S85.) 



An abstract of the Proceedings of the late Geological Congress at Berlin 

 has been published by the writer in Science ; a fuller report is about to 

 appear in the American Journal of Science and Arts. The report, contain- 

 ing all the documents relating to the work of the Congress, and only less 

 complete than the official report, will be presented to the American com- 

 mittee whenever it meets. In the meantime, it will interest Geologists 



