NATURE 



[March 2, 1893 



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 free from the considerable 

 absorption of the more refrangible rays which sparks pro- 

 vide in great abundance, and which are largely absorbed 

 by glass. On the other hand the photograph 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. 



Fig. 5 is a photograph of the apparatus set up in one 

 of the passages in i the Royal College of Science, in 

 which the experiments were 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 

 points/ in order that its position every time it is fired may 



through these holes is not diffused in any harmful manner. 

 The large box at the back is a case 5 ft. long, filled with 

 bran which stops the bullets gently without marking them. 

 The little condenser is just below the rectangular pro- 

 longation of the photographic box, the large condenser is 

 the vertical square sheet seen just to the right. The 

 electrical machine used to charge the condensers is seeji 

 on the table. It is a very beautiful 12-plate Wimshurst 

 machine made by Mr. Wimshurst and presented to the 

 Physical Laboratory. This machine not only works with 

 certainty but is so regular in its working that no 

 electrometric apparatus is necessary. All that has to be 

 done is to count the number of turns of the handle which 

 are required to produce the sparks at E and E' when the 

 gap at B is not joined, and to count the number which 

 are sufficient to produce a spark at E when the gap at B 

 is suddenly closed. Then if the rifle is fired after any 

 number of turns between these, but by preference nearer 

 the larger than the smaller number, the potential will be 

 right, the spark E, inside^the box, and the spark E', which 



Fig. 5. 



be the same. The bullet then traverses precisely the 

 same course, so that wires placed in the line between holes 

 in two cards made by one shot will be hit by the next. The 

 two wires which the bullet joins as it passes by are setup 

 in the box seen in the middle of the figure with the lid 

 propped up so as to show the interior. The photographic 

 plate is on the left-hand side and the spark when made 

 is just within the rectangular prolongation on the right- 

 hand side. Paper tubes with paper ends are placed 

 on each side of the box to allow the bullet to enter and 

 leave, and yet not permit any daylight to fall directly 

 on the plate. All is black inside, and so the 

 small amount of hght which does enter the box 



1 Six independent points of support are required for .1 geometrical clamp. 

 In this case a V support near the muzzle supplied two, a V support near the 

 breach two more points, the rifle was pressed forward until a projection under 

 the muzzle rested against the front V, thus allowing freedom of recoil, but 

 otherwise preventing all uncertainty of position except that due to rotation 

 in the Vs which is made impossible by the sixth point, that is, the lower end 

 of the stock resting sideways against a leather covered wooden bracket 

 fastened to the same table to which the Vs were attached. 



NO. I 2l8, VOL 47] 



is in sight outside the box, will be let off, and if a 

 plate is exposed a photograph will be taken. If by chance 

 the E' spark is not seen then there is no occasion to 

 waste the plate, another bullet may be fired after 

 resetting the wires and the result will be as good as if one 

 shot had not failed. When all is in order a failure of this 

 kind is very rare. I also arranged a tube in the side of 

 the box with a pocket telescope fixed in it and focussed 

 on the wires. If a piece of white card or paper is placed 

 in the line of vision and so as to be illuminated by a 

 spark let off as above described but preferably much 

 nearer the card, the bullet will be seen by any one looking 

 through the telescope. I took this down, however, at 

 once, as the photograph showed more than could ever 

 be seen by the eye. The box seen just to the right of the 

 rifle with a coil of wire upon it is the one in which the 

 revolving mirror was fixed, and in which the trails of 

 sparks made near the door at the end of the passage 

 were photographed. The apparatus for photographing 



