TRANSACTIONS OF THE SECTIONS. 



31 



uniform calibre. It is about 15 inches long, by £ inch diameter, and is marked 

 by Geissler as containing phos. hydrogen. As you have 

 perhaps observed, it gives the strata with extraordinary di- 

 stinctness ; and after the action has been continued a little 

 while, the strata near the blank end arrange themselves in 

 pairs, consisting each of a bluish and a more reddish layer, 

 separated by a blank interval from the next, as seen very 

 plainly in the photograph. 



By a steady, rapid motion of the ratchet-wheel of Mr. 

 Ritchie's coil, it was easy to keep the strata almost perfectly 

 stationary. The picture was obtained with eighteen turns of 

 the wheel, each giving twelve sparks. With six turns a 

 tolerably clear picture was secured. 



You see that the unilluminated space at one end made no 

 impression, and that the intervals between the strata are also 

 as devoid of actinic as they are of luminous rays. 



O r\ o r 

 \-J XJ \J 



The picture of the winding tube, with bulbs, shows how 

 superior is the actinism of the faint blue light of the negative 

 end compared with the brighter and less refrangible rays of 

 the opposite bulb. 



The third photograph was 

 produced by the two slender 

 Geissler tubes, containing re- 



spectively N and C 2 . The 

 former was placed below the 

 latter as they were presented 

 before the camera, and the 

 current was sent through them 

 in succession. To the eye the 

 intense whitest light of the 

 C0 2 tube was more dazzling than the crimson colour- 

 ing of the other. Yet you will observe the picture 

 made by the latter is far the stronger of the two, as 

 indeed might have been expected from its more re- 

 frangible illumination. 



This photograph was produced with half a turn of 

 the wheel, that is, six successive flashes of the light. 

 I am unable to state the aggregate time of exposure to 

 the rays, as I have not yet ascertained the duration of 

 a flash. This I hope, with Mr. Ritchie's aid, to ac- 

 complish at an early day. But if we assume the time 

 to be as much as tenfold the duration of the electric 

 spark, as measured by Wheatstone, we should have 



less than „-j-j3r th of a second for the entire time which 



I believe that a single 



the light required for producing this intensely clear picture, 

 flash would suffice, but I have not yet made the trial. 



