140 ANNUAL OP SCIENTIFIC DISCOVERT. 



copies Avci'e roquired, as for illustrating a book. The toning bath 

 oontaiuod half an ounce of acetate of soda to one 2)int of water, 

 and one grain of eliioridt^ of gold for each sheet toned. The 

 I)ieture was fixed witli hvposuli)hite of soda (eight ounces to the 

 l)int), and well washed with water. 



Much conversation then took place on this paper, in the course 

 of whii-h ]\Ir. Bcadic urged the employment of waxed paper, 

 instead of albunuiiized, as likely to give a more transparent 

 negative, and spoke of the ajiplication of carbon-printing to this 

 process. Mr. Brightman suggested the use of a green instead of 

 a black pigment in that method, to give the natural color of the 

 plant. 



PHOTOGRAPHING CANNON BALLS. 



Some months ago, when on a visit to Woolwich Arsenal, we 

 were shown Ijy Mr. M"Kiiday, Proof Master, some iihotograjihs 

 taken of guns while being lired, which not unnaturally excited 

 feelings of surprise. So rapid had been the exposure, and so 

 well had the i)ropcr moment for tin; cx])0surc been seized, that 

 the projectile couhl be seen jjrotrnding from the cannon's mouth 

 Avhile in the act of proceeding on its distant mission. Mr. M'Kin- 

 lay kindly alVorded us every requisite information relative to his 

 invention for securing such won<lerful results; and, from the fact 

 that tiie com2)arative eiru-ii'nc\- of certain kinds of small-arms, and 

 the intluence they are now exercising in European affairs, are at 

 present receiving a large share of public attention, we think that 

 it may not prove uninteresting to luring before our readers some 

 matters of scientilic interest in connection with our own " great 

 guns," and the means employed for ascertaining by photograjjhy, 

 and with the utmost possiljle precision, not only the jiath of a pro- 

 jectile in the air, but the time occu^jied in its progress between 

 two or more points anywhere in the course of its tlight. It will 

 be obvious that, when it is desired to ol)tain a photograph of a 

 gun at the moment of discharge, the gun itself must be made sub- 

 servient to the exposing and coveiung of the sensitive plate. It is 

 impossible that any person, however delicate his eyes and ears 

 may be, can operate so dexterously as to stop the exposure when 

 the ball has been projected, say a few inches from the muzzle of 

 the gun, and when it is consequently travelling at its greatest 

 velocity. This can only be accomplished by automatic arrange- 

 ments, aided by electricity. 



Let us now suppose that a stereoscopic camera, fitted with pow- 

 erful lenses of short focus, has a thin, light disk fitted up in front 

 of the lenses, revolving on an axis between the two lenses. Two 

 holes in this disk correspond with the apertures of the lenses, so 

 that if a circular spring — like that of a pair of snuffers — cause 

 the disk to make half a revolution with great rapidity, the holes or 

 apertures will, when flashing i^ast the apertures of the lenses, 

 admit the light for an exceedingly brief jjeriod of time. This is 

 the means employed in the arsenal for effecting the exj)osure of 

 the plate. 



We shall now enter into the details of the manner of discharg- 



