PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 443 



who has tried it may not be uninteresting to the readers of this Journal. 

 Photography, with the discovery of the use of collodion, seemed to leap 

 into its present high position at one bound, at least so far as the chemistry 

 of the art is concerned. The negatives of to-day look like the negatives 

 of the first experimenters, and the chemical, process of their production is 

 essentially the same. But with the optics, of photography the case is 

 different — here there has been a steady improvement. The wants of the 

 portraitists have been met by tlie construction of new objectives suited to 

 the style of pictures to be produced. In these instruments depth of field 

 with free admission of a large volume of light was what was most sought 

 for. Theory could not dictate what shape or combination of lenses would 

 best produce this result, and patient experiments were resorted to. The 

 requirements of landscape photography are quite different from those of 

 portraiture. A portrait tube may be used to take views if it be provided 

 with a stop or small opening to limit the amount of rays passing through 

 it and thus to deepen the field, or increase the ' reach' of the instrument as 

 it is technically called. This involves loss of light, and consequently 

 diminishes the quickness of its working. We hear continually of rapid or 

 instantaneous photography, and are often led to believe that the rapidity 

 is to be ascribed to some wonderful sensibility of the chemicals used, but 

 this is only partially true, and to the optician is due the most of the merit 

 of instantaneous pictures. A portrait tube with its full opening will, in a 

 sky-light room, produce a picture in perhaps ten or fifteen seconds. This 

 same instrument, with the satrie opening and same chemicals, exposed to 

 an extended view in bright sunlight, could not be opened and shut quick 

 enough; the immense volume of light reflected from so large an area of 

 space being concentrated on the same sized plate as in the first case, would 

 be too violent in its action, and from the nature of the instrument near and 

 distant objects could not be brought into focus at the same time. 



The human eye, when the head is at rest, takes in an angle of view of at 

 least seventy or eighty degrees, the whole of which is not seen clearly at 

 once, but can be examined in detail by the almost unconscious rolling mo- 

 tion of the eye in its socket — the actual included angle of clear vision at 

 any one instant being only one or two degrees. Hence a picture of a land- 

 scape, for instance, to fill the eye and seem a true representation of nature, 

 should include an angle of at least sixty degrees. Ordinary instruments, 

 such as have heretofore been used, do not include an angle of more than 

 one-half this amount, and hence has originated the complaint that photo- 

 graphic views represent mere patches of scenery and not pictures. I 

 remember once standing on a bridge — camera in hand — and looking up the 

 romantic Wissahicon. Tlie picture presented to my .eye was very beauti- 

 ful — the centre, a waterfall, framed in on both sides by wild and rugged 

 rocks, and spanned above by the arch of a railroad bridge crossing at the 

 tops of tlie cliffs. The foreground was made up of a stony bed, where 

 danced and foamed the rapid current. I planted the camera and hoped 

 Boon to peel off from this charming view a cuticle (as Dr. Holmes says), 

 which like plates of mica could be split and resplit for the collections of 

 my friends. But on the ground glass I found nought but the tumbling 

 water. No rocks, no bridge, no stony river bed — the poor camera in its 



