102 TRANSACTIONS OF THE [FEB. 10, 



moves away from the centre in the opposite direction. It was 

 assumed that a bright star would afford a similar halo with 

 suflBcient exposure. This led to the suggestion that photo-tele- 

 scopic lenses should be rigorously investigated in this respect, 

 and that it should be demonstrated that such objects as the 

 celebrated nebula in the Pleiades (which is revealed only by the 

 photographic plate) are neither wholly nor in part an aggregate 

 of such reflections from the group of stars within the field of 

 view. 



Photographs of Geissler tubes taken by exposures not exceed- 

 ing five seconds in duration, and showing the stratified discharge 

 in nitrogen, hydrogen, and carbonic anhydride, were next pre- 

 sented. The stratification was best obtained by a number of 

 separate flashes made at about half-second intervals by breaking 

 the primary current by hand. These were followed by other 

 photographs of a Crookes' tube, showing the mean free path 

 of radiant matter, in which the different phenomena resulting 

 from reversed polarity and shifting the connections, were dis- 

 tinctly portrayed. The photographing of the high tension 

 discharge in these various vacuum-tubes appeared to afford the 

 following conclusions : 



1. The bands of the stratified discharge seem to recur more 

 nearly at the same points of origin when the primary current is 

 broken at moderate intervals. 



2. Either variety of discharge affords an impression with a 

 Cramer (s 40) plate in five seconds or less ; hence but a short 

 exposure would probably be required to photograph not only 

 the form but also the detail of polar auroras. The reduction 

 by the lens, and consequent concentration of the light, would 

 probably lessen even this short exposure. 



3. If comets are foci of radiant matter," they may be photo- 

 graphed by short exposures. 



Divers series of lantern views and negatives next followed, 

 showing the effects obtained in several similar cameras sim- 

 ultaneously exposed, on the same self-luminous subject, but 

 charged with various plain and ortho-chromatic plates. Car- 

 butts' eclipse and eclii)se ortho-chromatic plates made of the 

 same emulsion ; imported and American eoside ; and plain 

 Cramer (s 40) plates were employed. The spectra of barium 

 and strontium, fire-works, and Geissler tubes afforded subjects. 

 Cramer plates unstained developed the strongest impression of 

 the latter subjects. In case of the spectra, the lines of the 

 barium spectrum between e and a affected the stained and not 



^ Papers of the A.merican Astronomical Society, No. 2 ; Brookl5^n, 

 N. Y., March, 1887, page 54. 



