308 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



produced by the combustion of magnesium. The invention of magnesian 

 lamps by Sir David Brewster and the improvements which were made 

 in them by M. Le Eonx, in mixing zinc with the magnesium, have 

 facilitated the application of this artificial light to photography. 

 Engravings, busts, and statues were first attempted, and thus was 

 recognized the photogenic value of the magnesian light, which more- 

 over is less costly than the electric light. 



By this method, inanimate objects may be advantageously repro- 

 duced ; but, in an artistic point of view, the effect is unsatisfactory on 

 account of the necessarily exaggerated contrast of light and shade. The 

 photographic portraits by magnesium have a cadaverous appearance. 

 On the other hand, we have obtained images of objects which w r ould 

 otherwise have escaped the photographer's art ; for example, the inte- 

 rior of one of the pyramids of Egypt, and of the celebrated caverns of 

 Kentucky, known as the Mammoth Caves ; the magnificent stalactites 

 of those subterranean rocks have thus been reproduced with the 

 utmost fidelity. Subterranean curiosities, like the catacombs of Eome, 

 and those of Paris, have also benefited by this mode of illumination. 



Other aitificial lights have been tested, with more or less success, 

 for the production of photographic images. Such are the lights pro- 

 duced by the combustion of a jet of oxyhydrogen gas, projected 

 on to solid fragments of refractory matter : lime, magnesia, zirconium, 

 chromium. Van Monckoven has obtained enlarged proofs on 

 collodion or on paper, in a space of time varying from one to three 

 minutes, by the light of the gas blow-pipe projected on a mixture of 

 titanic acid, magnesia and carbonate of magnesia. It is not, after all, 

 so much the luminous intensity of the source which is favourable to 

 the reproduction, as the quantity of chemical rays emitted. 



VI.- ENLARGED PROOFS. MICROSCOPIC PHOTOGRAPHY, 



It is evident that by projecting, with the aid of a solar microscope, 

 the image of a phonographic proof on a sensitized surface, an image 

 will there be formed with all the details of the original enlarged. It 

 this be done with a negative, the result will be a positive ; but an 

 enlarged negative may also be obtained, and as many positives as are 

 required may be obtained by the ordinary means. This last process 

 is much more expeditious, and is as follows : 



