1896.] "^^ 



have been examined by means of the Routgen process, and it may be 

 interesting if one of your members will open the envelope and compare 

 the description therein, with the one that will now be detailed. The 

 picture tells its own story pretty well, even to the uninitiated. No. 1 

 seems to have three circular holes, plugged up with some substance, 

 doubtless aluminum, having the same radiographic density as the 

 material of the plate. No. 3 appears to be perfect. No. 3 has two 

 holes similar to those of No. 1, and a third stopped up through a por- 

 tion of its length by some substance les? transparent than the aluminum, 

 perhaps a piece of copper or iron wire (Dr. Richards' Description of 

 Aluminum Plates). 



Our experiments during the last two weeks have been made at all 

 times of the day and evening, sometimes in full daylight, and often 

 with no light at all, except that emitted by the tube. The presence or 

 absence of luminous radiation seems not to make the least ditference in 

 the results. We early learned that sharper outlines could be obtained 

 by omitting the usual plate holder, and wrapping the plate in several 

 thicknesses of orange paper. By this means actinic light was excluded, 

 and the objects were brought nearer to the sensitive film. During this 

 series of experiments, the writer has received much assistance and man}' 

 valuable suggestions from his associates in the department, Dr. H. C. 

 Richards, Dr. R. R. Tatnall and Mr. G. C. McKee. 



In connection with this subject, it is desired to direct the attention of 

 the gentlemen present to a remarkable coincidence which can hardly 

 fail to excite interest. In the fall of 1889, the writer received a letter 

 from a prominent gentleman in Philadelphia, asking him to call at a 

 convenient and early date, to be presented to a friend who was desirous 

 of obtaining facilities for some experiments in electric spark photog- 

 raphy. On the occasion referred to, the writer had the pleasure of 

 meeting Mr. W. N. Jennings, of Philadelphia, who for many years has 

 been much interested in the photography of lightning. It was Mr. 

 Jennings' wish to photograph electric sparks from various forms of ap- 

 paratus, in order to compare the results with the lightning pictures 

 which he had already obtained. It is needless to say that the series of 

 experiments, begun at that time, have been continued to the present, as 

 occasion and opportunity have made it convenient. 



The particular meeting of interest occurred on the evening of Feb- 

 ruary 32, 1890. Slides 11 to 14 show the result of some of our experi- 

 ments on that evening. We photographed the brush from a large in- 

 duction machine, by holding the uncovered plate in various positions 

 near the poles. We also placed coins and brass weights on the plates, 

 sparking them by means of the Apps induction coil in various ways. 

 After finishing the experiments of this sort, the writer brought out from 

 the cabinet quite a variety of Crookes tubes, and showed them to Mr. 

 Jennings simply for his pleasure and amusement. The desirability of 

 getting Mr. Ives to reproduce some of the color effects by means of his 



