j873-J Actinism and Magnetism. 295 



a rich purple. This actinic action on the juices of plants 

 has not been deeply investigated, nor has it as yet been ap- 

 plied to any practical purpose. 



Another example occurs in the animal kingdom. The 

 common earwig, if reared in the dark, is almost colourless, 

 being of a nearly uniform creamy white ; but if it be sub- 

 sequently exposed for some hours to moderate daylight, it 

 will eventually acquire its natural dark colours. 



It appears to be part of the wise dispensation of Divine 

 Providence in the government of the human race, that the 

 most useful discoveries should be made only after the 

 exertion of a great amount of industry, applied with much 

 wisdom and skill — and that, too, not by a single individual, 

 but by a long succession of men. It is given to one to dis- 

 cover a principle, to another to take advantage of it for the 

 attainment of some practical end, to a third, a fourth, and 

 a fifth to make successive improvements in the working out 

 of the principle, and in modifying its mode of action. Thus, 

 Niepce first discovered the effects of light on films of bitu- 

 men ; this result suggested to Daguerre the application of 

 iodine vapour to produce on plates of silver a film sensitive 

 to light, and the subsequent development of the image by 

 mercurial vapour — a photographic process which bears the 

 name of its inventor, It was Scheele's discovery of the 

 action of light on chloride of silver, followed up by Wol- 

 laston, that led Fox Talbot to its practical application in 

 obtaining photographic images on paper, and to his further 

 discovery of the mode of producing a latent photographic 

 image on iodide of silver capable of subsequent development 

 by the application of a powerful deoxidising agent. These 

 results paved the way for Archer, who availed himself of 

 Schcenbein's discovery of soluble cotton or collodion to 

 spread a film of that substance on glass, and charge it with 

 iodide of silver, so obtaining a more sensitive and manage- 

 able medium for the reception of the latent photographic 

 image, to be afterwards subjected to the action of a de- 

 veloping agent. Other and later labourers in the field have 

 greatly improved on those earlier methods, until the taking 

 of pictures by means of salts of silver and developers has 

 now reached a pitch of perfection of which the earliest 

 pioneers in the art had scarcely dared to dream. 



To the lot of the author it fell to discover the photo- 

 graphic properties of the double salts of chromic acid when 

 in contact with organic matter, and the curious fact that 

 the disengagement of the chromic acid from the salt under 

 the action of light, and its immediate re-combination with 



