364 SCIENTIFIC AMUSEMENTS. 



other photographers, dry plates are now extensively used, and are 

 manufactured on a large scale at prices so low that, excepting for 

 special purposes, it is hardly worth while to prepare them oneself. 



A collodion plate prepared as in the last experiment is practi- 

 cally a film of organic material through which particles of silver 

 iodide, or iodide mixed with bromide, are disseminated, these latter 

 being precipitated in the body of the film after its application to 

 the supporting plate. Just as iodide and bromide of cadmium, &c., 

 can be added to collodion to "iodise" it, thus impregnating it 

 with these salts ; so, similarly, can a solution of nitrate of silver in 

 alcohol be also added, with the result of precipitating in the body 

 of the fluid particles of insoluble silver iodide and bromide, which 

 thus form with the fluid a sort of " emulsion " ; when this is poured 

 out on a supporting plate a film containing particles of suspended 

 silver compounds results, very similar to that obtained in the last 

 experiment, chiefly differing in that, as the silver salt particles are 

 already present in the film, no immersion in solution of silver 

 nitrate is requisite. Dry plates of this kind are not very sensitive 

 to light, and hence are but little used; but .plates prepared in 

 somewhat similar fashion, substituting a solution of gelatin in 

 water for one of guncotton in mixed ether and alcohol, are largely 

 employed, the sensitiveness to light being capable of being rendered 

 extremely great by appropriately arranging the details of the 

 manipulation, more especially by the careful application of heat 

 to the gelatin emulsion containing silver bromide, either without 

 admixture of iodide or containing only a small proportion thereof. 



Expt. 389. Instantaneous Photographs. The chemical action 

 of light upon most substances requires to be exerted for some 

 amount of time before measurable amounts of change are pro- 

 duced; but gelatin bromide plates prepared in particular ways are 

 so extremely sensitive to light that an exposure for only a very 

 small fraction of a second in the camera is sufficient to produce 

 the required effect. Accordingly, by employing an instrument so 

 constructed that light is admitted for only an extremely short 

 interval of time (an orifice being rapidly uncovered and covered 

 again by a rapidly moving slide, &c.), it is possible to take instan- 

 taneous photographs, i.e., photographs where objects really in 

 motion are depicted as though at rest, the time of exposure being 

 so brief that no material change of position has occurred during that 

 short period ; thus a railway train in quick motion, a wave break- 

 ing on rocks and throwing up clouds of spray in a storm, the flight 

 of a flock of birds in the air, &c., may be thus clearly and distinctly 

 photographed; or what amounts to much the same thing, an 

 operator may take a photograph of the surrounding objects whilst 



