BECEEATIVE SCIENCE. 



189 



upon the other, the tinder leaf presenting a 

 beautiful photograph of the upper one, its 

 serrated edge and form being perfectly de- 

 fined. Wherever the shade was cast, that 

 part of the leaf was of a deep green, while 

 the unshaded parts were of a pale sea-tint. 



FiBST Peinciples or Photogeapht 

 Enown to the Alchemists. — The action 

 exercised by light upon fused chloride of 

 silver, horn-silver, is alleged by Dr. HaUeur, 

 of Berlin, to have been known to the alche- 

 mists as early as the 16th century, who noted 

 that light imparted a black tint to the ori- 

 ginally white salt of silver. The chloride 

 of silver suffers under the influence of the 

 prismatic rays. 



The Phenomena Noticed by Subse- 

 quent ExpEEiMENTEES. — Petit published 

 his observations upon the influence of light 

 on the crystallization of various salts in 1722, 

 and Chaplot and Dieze in 1788 and 1789 ; 

 and Scheele, in 1777, published his observa- 

 tions on the nigrescence of chloride of silver 

 under the influence of light, and on the 

 alterations. 



Silhouette Piguees Taken by the 

 Action of Solae Light. — One of the ear- 

 lier chemists, M. Charles, is recorded, about 

 this period, to have exhibited, in his lectures 

 at the Louvre, a paper capable of taking sil- 

 houette figures by the action of solar light, 

 but no account has come down of his process. 



Wedgewood, the Pibst Photogba- 

 phee. — The honour of being the first practi- 

 cal photographer is generally conferred upon 

 Wedgewood, who, in 1803, contributed a 

 paper to the Journal of the Royal Institution, 

 accompanied by observations from Sir Hum- 

 phry Davy, entitled '* An Account of a 

 Method of Copying Paintings upon Glass, 

 and of Making Profiles by the Agency of 

 Light upon Nitrate of Silver." White paper, 

 or white leather, saturated with a solution of 

 nitrate of silver, served as the impressionable 

 surface. Wedgewood's description of his 

 discovery is tPQ yemarkabje i^ot to he re- 



corded. He says : — " The alteration of the 

 colours commences the more speedily in pro- 

 portion to the degree of the intensity of the 

 light. In bright sunshine, from two to three 

 minutes sufiice to produce the full effect; 

 whereas, in the shade, several hours are re- 

 quired to arrive at the same effect. Through 

 differently coloured glasses the light acts with 

 different degrees of intensity. When the 

 shadow of a figure is thrown upon the pre- 

 pared surface, the parts covered by the sha- 

 dow remain white, whilst the other parts 

 speedily turn black. To copy paintings on 

 glass, the negative images should be taken 

 on leather, because the action is more rapid 

 with this material than if paper were used. 

 When the colour is once fixed on leather or 

 paper, it proves fast to a degree ; water, and 

 even soap and water, failing to remove it. 

 Besides the method of copying just given, 

 there are several others. It will be useful to 

 make copies of all such objects as are partly 

 transparent and partly opaque. The woody 

 fibre of the leaves of plants and the wings of 

 insects may be very accurately copied in this 

 manner. All that is needed to this end is, to 

 transmit the direct light through the object 

 to be portrayed, and to receive the shadow 

 on prepared paper or prepared leather." 



EXPEEIMENTS BY SlE HuMPHEY DaVY. 



— Sir H. Davy, in commenting on Wedge- 

 wood's discovery, says : — " It has been ob- 

 served that the image of the camera obscura 

 is too feeble to make an impression upon the 

 nitrate of silver within a reasonable space of 

 time. To copy these images was Wedge- 

 wood's chief object. I followed up his expe- 

 riments, and found that the images of small 

 objects produced by the solar microscope may 

 be copied without difficulty on prepared paper. 

 A comparison of the effects produced by the 

 action of light upon chloride of silver, with 

 those produced on nitrate of silver, fully and 

 clearly showed that the chloride is the more 

 sensitive compound of the two. All that is 

 required now to render these experiments as 



