29 S Actinism and Magnetism. [July; 



exposed under the negative for three or four times the 

 period required for an ordinary picture. A plate of glass, 

 thoroughly cleansed, having been gently warmed, receives a 

 thin equable coating of Scehnee varnish. When this is dry, 

 the picture is transferred from the black gelatinised paper 

 to the glass plate, under luke-warm water in the usual 

 manner. The picture is then to be washed clean, and 

 allowed to dry thoroughly. A margin of very thin paper 

 having been applied all round it, a second very clean thicker 

 glass plate is to be laid over the picture, and carefully ce- 

 mented to it all round the edges. The picture is thus 

 enclosed between the two plates. The back glass (the 

 thicker of the two) must then be coated with Brunswick 

 black all over the background of the picture, the outlines 

 of which must be carefully traced, so that no light may 

 penetrate between the picture and the background. When 

 the black varnish is quite dry, the picture is to be placed 

 at an angle of 45 degrees, with a piece of mat gilt paper 

 below it. When the transparency is thus viewed by the 

 light reflected from the gilt paper, it presents the appearance 

 of a bas-relief. This effect is so decided, that the spectator 

 can hardly persuade himself that he is looking on a flat 

 surface. 



While the attention of investigators has thus been for a 

 considerable number of years past directed almost exclu- 

 sively to ascertaining the best means of rendering the 

 actinic properties of light available lor practical purposes, 

 and rightly so, it is not well that the theoretical questions 

 connected with actinism should be entirely neglected ; for 

 a thorough search into the principles and modes of actinic 

 action is the most promising way of arriving at results 

 which may eventually prove of further practical utility. 



One of the earliest and most interesting questions which 

 presented itself to the inquiring mind, was the possibility of 

 explaining actinic action in accordance with the principles 

 of the undulatory theory of light. In a former work by the 

 author, the first edition of which was entitled '"'The Material 

 Universe/' and the second was (much against his wish) 

 entitled ''The Great Architect," he indicated the manner 

 in which the formation and subsequent development of the 

 latent photographic image, both in the process of Daguerre 

 and in that of Talbot, might be explained agreeably to the 

 undulatory theory-. A further development of his views was 

 subsequently published in " The Engineer," and again 

 briefly re-stated in the notes of his more recent work, en- 

 titled '•' The Beginning; &c." But as his ideas have thus 



