CHAP, vi.] . PHOTOGRAPHY. 289 



CHAPTER VI. 



PHOTOGRAPHY. 



I. FIKST ATTEMPTS AT FIXING THE IMAGES PRODUCED IN THE 

 CAMERA OBSCURA DISCOVERIES OF NIEPCE AND DAGUEKRE. 



WHEN rays of light, proceeding from an object, are received on a 

 white surface, at the focus of the converging lens of the camera 

 obscura, a marvellously faithful image is produced. It is a true 

 picture in miniature of the landscape in view, with all its shades of 

 light and colour and all the most minute details ; but it is a fleeting 

 image, quite ideal, so to speak, consisting only in the movement of 

 the waves of light. We close the opening which gives access to 

 those waves, and, instantly, the image vanishes. 



More than one observer, from Porta, the inventor of the camera 

 obscura, to Niepce and Daguerre, the inventors of photography, must 

 have desired to retain and fix these images, and thus to enlist nature 

 herself as coadjutor with art in drawing and painting. What was 

 required to produce this result ? The knowledge of another property 

 which the rays possess, of acting chemically on certain substances, 

 and thus leaving a visible trace of their action, the power of which 

 is in proportion to the intensity of the rays. In 1770, Scheele had 

 discovered the property possessed by chloride of silver of turning 

 black under the influence of light, or rather he had studied afresh 

 this action known to the alchemists of old. It was by utilising this 

 property that an able French naturalist succeeded, in the early part 

 of this century, in obtaining sketches by the action of light. We 

 do not know how he obtained them, but doubtless the process em- 

 ployed by him had some analogy with that described by Arago in the 

 following terms, and by which negative proofs of a picture may be 



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