MATHEMATICS AND NATURAL PHILOSOPHY. 165 



no impression is produced. It is the same if a sheet of mica be substituted for 

 the glass, or a sheet of rock crystal, or a yellow glass coloured with the oxide of 

 uranium. An engraving covered with a layer of collodion or gelatine is repro- 

 duced but not if it be covered with picture-varnish or gum. 



An engraving placed at a distance of three millimetres from the prepared paper 

 is reproduced very well, and if it be a bold design, it is reproduced at a distance 

 of one centimetre. The reproduction therefore is not the result of contact or 

 chemical action. All the parts of an engraving colored with different colors are- 

 not produced with the same intensity ; they vaiy according to the chemical com- 

 position of the colors : the same may be said of different kinds of inks. Vitri- 

 fied characters traced upon a plate of varnished or enamelled porcelain are repro- 

 duced, but the porcelain leaves not the faintest trace ; but if the porcelain be free 

 from varnish or enamel, it leaves an impression, though a faint one. 



If, after having exposed an engraving to the light for an hour, it is placed in 

 contact with a piece of pasteboard which has been kept in a dark place for some 

 days previously, and at the expiration of not less than twenty-four hours the 

 pasteboard be brought into contact with the prepared paper, and allowed to remain 

 thus for another twenty-four hours, the result will be the reproduction of the en- 

 graving — a little fainter, it is true, than if the engraving had been applied directly, 

 but still distinct. Likewise if a tablet of black marble dotted with white spots 

 be exposed to the light, and then applied to the prepared paper, the white spots 

 alone will produce an impression. Under the same conditions, a tablet of white 

 chalk will leave a visible impression, whereas one of black charcoal makes none 

 whatever. A black and white feather exposed to the sun, and applied in the same 

 manner, produced the same results. 



The author drew particular attention to the following experiment, which is per- 

 haps the most curious and the most important. He took a metal tube (any 

 other opaque substance in the form of a tube answers the same purpose) closed at 

 one of its exti*emities, and Hned with white paper or cotton, and exposed the open 

 end to the direct solar rays for about an hour ; after the insolation he applied the 

 same end to a sheet of prepared paper, and found after the lapse of twenty-four 

 hours that the area covered by tlie tube had been darkened. More than that, an 

 engraving on Chinese paper interposed between the tube and the prepared paper, 

 was itself reproduced. 



If the tube be hermetically closed immediately on being withdrawn from the 

 light, it will preserve for an indefinite period the power of radiation communicated 

 to it by insolation. 



A piece of white 'card placed in a dark room into which an image vividly 

 illuminated by the solar rays was thrown, was found after a three hours' exposure 

 to- give a faint representation of the object upon the prepared paper, after twenty- 

 four hours' contact. 



It remains only to speak of the result (jf experiments made with fluorescent 

 and phosphorescent bodies : — A design ti'aced upon a sheet of paper with a solu- 

 tion of sulphate of quinine (one of the most fluorescent bodies known,) and 

 exposed to the sun, then placed in contact with tte prepared paper, will reproduce 

 itself in a much more intense black than the paper on which it is traced. A sheet 

 of glass, however, placed between the design and the pre23ared paper, prevents 



