VII. PHOTOGRAPHY. 243 



tortion of image is not entirely avoided, it is perhaps counterbalanced by the 

 increased brilliancy of image and equality of illumination, due to the absence 

 of all disturbing reflex images. 



4. A Wide angle rectilinear lens of 1|" diameter, and V equivalent focal 

 length for 12" x 10" pictures. 



This lens is constructed upon the same general principle as the symmetrical 

 3" combination already described, with this difference, that the lenses com- 

 posing it, though similar in form, are of proportionately smaller diameter, 

 i.e., they are thinner, and being placed nearer together, they transmit more 

 oblique pencils, and a larger angle of picture is included, as much as 90, with 



apertures ^ to 4 ~. 



This instrument is designed for photographing objects in confined situations, 

 such as interiors of buildings, monuments, &c., where the camera cannot be 

 removed to more than a given distance. It produces images free from distor- 

 tion and flare. 



962a. Apparatus for producing photographs in permanent 

 pigments ; consisting of, 



Registering pressure frame. 



Plates of zinc, porcelain and opal glass (the plates ha\ 7 ing upon 

 them : a. The sensitive tissue ready for development, b. The 

 supporting paper stripped off.' c. The picture partly developed. 

 d. The picture ready for transfer). 



Reservoir for hot water, with means of keeping it warm, and 

 grooves for the plates. 



Zinc trays for development washing, &c. 



Wooden stool and squeezer for the mounting of the exposed 

 sensitive tissue. The Autotype Company. 



If a negative in half-tones be placed under a paper coated with gelatine 

 pigments of any kind, and potassium dichromate, and then be exposed to 

 light, the gelatinous film is rendered insoluble, to depths varying according 

 to the intensity of the light passing through the various portions of the 

 negative. If the paper were at this stage exposed to the uction of hot wnter^ 

 it would be found that the soluble portions lay between the exterior surface 

 of the film and the inner surface of the paper. In order to develop the 

 picture, it is transferred by simple atmospheric pressure to a zinc plate or 

 other temporary and impervious support. The original paper peels off, and 

 the development takes place by the simple application of hot water, the shades 

 of the image being formed of different thickness of gelatine and pigment. 

 The image may be retransferred from the temporary support, or if developed 

 on a paper support may be left as it is, in which case, unless the negative be 

 reversed, the picture will appear reversed. 



962 c. Sir John Herschel's Actinometer, by Robinson, 

 marked I and K. 



The Meteorological Committee of the Royal Society. 



977a. Actinometer. Prof. Balfour Stewart. 



962b. Actinometers, or instruments for ascertaining the 

 intensity of the action of the light, and by which the exposure of 

 the sensitive pigment paper under the negative is regulated. 



Johnson's, Vogel's, .Spencer's, Lambert's, Sawyer's, Burton's, 

 Vidal's. The Autotype Company. 



Q 2 





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