ELECTRIC-SPARK PHOTOGRAPHS. l71 



a plate of window glass with a square foot of tiu foil on either side. 

 This condenser is charged until its potential is not sufficient to make a 

 spark at each of the gaps E, and E', though it would, if either of these 

 were made to conduct, immediately cause a spark to form at the other. 

 c is a Leyden jar of very small capacity connected with C by wire, as 

 shown by the cor.tinuous lines, and by string- wetted with a solution of 

 chloride of calcium, as shown by the dotted line. So long as the gap 

 at B is open this little condenser, which is kept at the same potential 

 as the large condenser by means of the wire and wet string, is similarly 

 unable to make sparks both at B and E', but it could, if B were closed, 

 at once discharge at E'. Now, suppose the bullet to join the wires at B, 

 a minute spark is made at B and at E' by the discharge of c, immediately 

 C, tinding one of its gaps E' in a conducting .state, discharges at E, 

 making a brilliant spark, which casts a shadow of the bullet, etc., ujion 

 the photographic ]»late P. Though this is simple enough, the ends 

 that are gained by this contrivance are not so obvious. In the tirst 

 place the discharge circuit of C, via E and E' is made of very short 

 broad bands of copper, a form which favors both the brilliancy and the 

 shortness of duration of the sparks; further, the doul)le gaj), of which 

 E' may be the longer, causes the intensity of the light of either spark to 

 be greater than it would be if the other one did not exist — in a particular 

 case the light of the shorter was increased six or eightfold — at the same 

 time the duration is not greatly affected. For this reason the spark 

 at E may be made very short, so that the shadow is almost as sharp 

 as if the light came from a point. The spark formed at B, which is 

 due to the discharge of c only, is very feeble, so that it is unable to act 

 on the plate, whereas, had the discharge of C been carried round by B, 

 tlie light at this ])()int would hopelessly have spoilt the plate, and at 

 tlie same time the liglit at E would have been feebler and would have 

 lasted longer. 



The wet string, while it is for the purpose of keeping the condenser 

 c charged a perfect conductor, is nevertheless, when this discharges at 

 E' and B, practically a perfect insulator; if it were replaced by wire, 

 then C would also wholly or partially discharge itself by B and E'. 

 Finally, in avoiding all lenses one is Iree from the considerable absorp- 

 tion of the more refrangible rays which sparks provide in great abun- 

 dance, and which are largely absorbed by glass. On the other hand, the 

 ])hotograj»h is a mere shadow; but this is no drawback, for the bullet 

 itself is on either system a mere silhouette, whereas the atmospheric 

 phenomena are more sharply defined and their character is more clearly 

 indicated without lenses than is possible when they are employed. 



Plate III is a }>hotograph of the apparatus set up in one of the pas- 

 sages in the Royal College of Science, in which the experiments M'ere 

 made. It is apparently of the rudest possible construction. The rifle 

 seen on the left of the figure is of course made to rest freely on six 



