NATURAL PHILOSOPHY. 139 



finds its natural place so completely that its existence would even 

 have been anticipated, had it not already been observed. 



On the other hand, substances that are merely sensitive to light 

 when brought into contact with others, but which have no power 

 of retaining light impressions until the decomposing agent is 

 brought into contact with them, are likewise incapable of receiv- 

 ing latent images. But these capacities may exist conjointly", as 

 we see in the case of a large number of silver compounds. 



This new view will, I think, dispel all the mystery that has 

 seemed to some to envelop the idea of a physical image, and 

 brings all the most obscure facts of photo-chemistry into parallel- 

 ism with well-understood and very simple phenomena. Phila- 

 delphia Photographer. 



i 



HELIOGRAPHY, OR SUN-ENGRAVING. 



Mr. Charles Negre, of Avignon, described his beautiful process 

 of chemical steel engraving before the Photographic Society of 

 Paris, at the last meeting reported in our foreign exchanges of 

 March 1st. The steel plate is first coated with a varnish of some 

 soluble substance mixed with bichromate of potash, which has the 

 property of becoming fixed, or insoluble in water, by the action 

 of light. This coating is then exposed to the wrong side of the 

 negative obtained directly in the camera, and the light transmitted 

 through the light portions of the negative fixes the varnish, while 

 the dark parts leave it soluble so far as covered by them. The 

 soluble portion of the varnish being washed off, the residue per- 

 fectly represents the lights of the negative, which are the shades 

 of the future picture. The plate is then placed in a gold bath and 

 submitted to the action of electricity, which beautifully gilds the 

 exposed parts of the surface with a layer of gold inseparable from 

 the steel, and distributed of course to the minutest points unpro- 

 tected by the fixed varnish. All that remains is to clean off the 

 fixed varnish and subject the surface to- the action of diluted acid, 

 which has no effect upon the gilded parts, but etches the exposed 

 surface of steel with a delicate exactness which no manual skill 

 could imitate. The plate is now ready to give impressions with 

 ink, although it will of course receive any desired additions or 

 alterations at the hands of the engraver. The process is of ines- 

 timable value both to the investigation and diffusion of science, 

 and with the aid of the microscope will introduce to common view 

 many of the most instructive and curious minutiae of nature. 

 Scientific American. 



LOXODROGRAPH. 



An ingenious contrivance, called the loxodrograph, has been 

 recently invented by a French naval engineer, M. Corradi, for as- 

 certaining a ship's course during a voyage. On the dial of the 

 compass, instead of the star which indicates the north, a circular 

 opening is made, furnished with a small lens. The light shining 

 upon the compass penetrates through the lens, and traces a black 

 mark or line on a sheet of sensitive paper underneath, which is 

 made to move at a certain speed by means of clock-work. The 



