LATENT IMAGE. 365 



he himself, together with his apparatus, is in quick motion (on a 

 coach, ship, railway train, &c.). 



Expt. 390. Development of Latent Image Negatives and 

 Positives. When photographs of the film class are obtained, it 

 usually happens that on examining the "plate," &c., that has been 

 exposed in the camera (Expt. 387) by the faint light of a sodium 

 flame (Expt. 355, practically devoid of chemical rays), absolutely no 

 visible trace of any action can be seen ; but that a change in the 

 silver compounds or other substances used has been effected is at 

 once demonstrated by the application of the "developer," ., a 

 fluid containing substances in solution capable of producing further 

 chemical action on those parts of the plate that have undergone the 

 incipient invisible action, to an extent proportionate to the extent 

 to which this invisible change has been effected; so that wherever 

 the light has acted most during exposure (i.e., at those parts of the 

 picture which are lightest, e.g., the shirt collar and front in the case 

 of a portrait), the developer produces most further chemical action, 

 and vice versa. The picture or image on the exposed plate is in 

 such cases said to be latent; and the action of the developer is to 

 produce a formation of minute particles of metallic silver by means 

 of a " reducing " action somewhat analogous to that occurring in 

 Expt. 120, where silver chloride is converted into spongy metallic 

 silver by means of caustic soda and milk sugar. As this action 

 goes on most rapidly where most light has fallen, it results that in 

 the "lights" of the picture most silver is thus reduced by the 

 developer, and in the shadows least ; hence, when the developed 

 picture is held up to the light (especially after "fixing" or dis- 

 solving out the remaining unaltered bromide or iodide of silver), 

 wherever the original light has been brightest, most silver is re- 

 duced, so that least light now shines through; i.e., when the picture 

 is viewed by transmitted light the lights and shadows of the object, 

 &c., originally photographed are reversed; such a picture is 

 spoken of as a negative. On the other hand, if the picture be 

 looked at against a black background, such as a coating of black 

 varnish or a piece of black cloth, &c., wherever least silver is 

 deposited, the black background is most clearly seen, and wherever 

 most is present the deposited metal appears white or light grey 

 by contrast ; in this case the lights and shadows are not reversed, 

 and the picture thus viewed is termed a positive. The quickly 

 produced cheap photographs executed by peripatetic photographers 

 on holidays, on the sands at the seaside, &c., are mostly positives 

 of this kind. 



A landscape photograph directly projected on a screen by a 

 magic lantern would accordingly give a negative picture. To 



