NATURAL PHILOSOPHY. 141 



ing and arresting the circular exposing diapliragm. The opening 

 and shutting of the camera at the j^recise instant of time is, as we 

 have said, by far too nice an operation to be accomplished by 

 hand. It must be borne in mind that a gun commences to recoil 

 as soon as the i^rojectile is fairly clear of its muzzle. The picture 

 which Ave examined had been taken when the projectile was yet 

 emerging from the gun's mouth, and before it had got quite clear 

 of it, and consequently before the recoil of the gun had com- 

 menced. The exposure was very rai;)id, but not so much so as to 

 show the front edge of the emerging projectile with a sharp out- 

 line. Although the gun, from the recoil not having commenced, 

 was quite sharp, the front edge of the projectile was, so to speak, 

 vignetted. 



The gun is fired by means of the galvanic tul^e invented by Mr. 

 M'Kinlay, and such as is used in pi'oving ordnance. Inside of 

 this there is a small platinum wire, which, when a current of elec- 

 tricity is passed thi'ough it, instantly becomes red hot, and melts. 

 Let us now see how this affects the operation of photographing 

 the gun. When the gun is ready for firing, the disk in front of 

 the lenses is wound up so that the rotating force of the spring in 

 the centre is at its maximum. It is retained in this position by 

 means of a catch and trigger, the latter of which is operated on 

 by means of an electro-magnet. The following, then, is what 

 takes place : When the galvanic current is sent through the wire, 

 the fine platinum wire imbedded among the gunpowder of the 

 discharging tube or fuse immediately becomes red hot, and melts. 

 But, while in jirogress of melting, it accomplishes two things ; it 

 transmits a current through it by which the electro-magnet be- 

 comes vivified and pulls the discharging trigger of the disk in 

 front of the camera lenses ; and secondly, it ignites the gunpow- 

 der and discharges the gun. But were this all, the exposure 

 would be made before the powder had had time to ignite and 

 consequently discharge the gun ; hence it is important that the 

 lenses be kept open until the gun really discharges its contents. 

 The means for effecting this are as simple as they ai'e ingenious 

 and complete. When the trigger acts so as to release the disk 

 from its enforced pent-up condition, it is pro^jelled forward by the 

 central spring until the apertures in the disk and those of the 

 lenses coincide, where, by means of a stop, the disk is retained 

 until the powder is ignited and the gun discharged, when, the 

 platina wire being ruptured, the passage of the electricity is 

 stopped, the electro-magnet simultaneously losing the power by 

 which it was enabled to arrest the rotatory progress of the disk, 

 which thus darts forward and closes up the camera as the con- 

 tents of the gun are in the act of being ejected from it. — British 

 Journal of Photography. 



STATISTICS OF PHOTOGEAPHY. 



The rapid gi'owth of new and special industries, says the "Brit- 

 ish Quarterly Review," is a fact so characteristic of the present 

 day, that the statistics of photography can scarcely be regarded as 



