is left behind. The glass is then plunged in a neutral 

 or very slightly acid bath of silver nitrate solution. In 

 about three minutes the salts in the film are converted into 

 silver iodide. The plate is exposed and the film is found to 

 be exquisitely sensitive, pictures being taken in the camera 

 in fewer seconds than minutes were required by the earlier 

 processes, the rapidity being further increased by the addition 

 of a small proportion of bromide. The exposed plate bears 

 no visible image, but if a small quantity of sulphate of iron 

 or pyrogallic acid (in solution) restrained by acetic or citric 

 acid, is poured upon it, the picture rapidly appears. It is 

 then fixed, as it is called, by washing with a solution of soda 

 hyposulphite, which salt has the very convenient property of 

 dissolving away the iodide or bromide of silver which has 

 not been acted upon by light, leaving in its place a more or 

 less transparent film, but not touching, at least for some 

 time, the photographic image. The picture is then dried 

 and varnished. This process is moderately rapid, and cannot 

 be surpassed for delicacy of the finished picture. It there- 

 fore at once supplanted the Daguerrotype and Talbotype. 

 Its great drawback arises from the fact that exposure and 

 development must immediately follow the sensitizing of the 

 plates, thus necessitating the transport of lightproof tents, 

 nitrate of silver baths, and other paraphernalia. It is 

 admirable for use in the home studio, but becomes almost 

 an impossibility for the peripatetic amateur. 



Let me pause here to explain the rationale of 

 " development." When an iodized collodion plate has 

 been taken from the bath and exposed, there is, as we 

 have said, no visible image. Nevertheless, the light re- 

 flected from the object has acted upon the plate and 

 reduced the iodide or bromide of silver to a subsalt in 

 proportion to its intensity. The nitrate of silver with 

 which the plate is still moist is reduced by the action 

 of the ferrous sulphate to the metallic state. But the 

 presence of the acid restrainer confines the deposit of silver 

 to the subsalt. Now we know that when one particle is 



