310 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



production. Others have contributed to improve these processes and 

 to obtain proofs of great perfection ; we may mention, among others, 

 M. Neyt (of Brussels), Messrs. Dagron, Moitessier, Lackerbauer, 

 Girard, in France. Every one knows those marvellous and imper- 

 ceptible photographs, portraits, views, monuments, &c., the size of a 

 pin's head, which, framed in the collet of a ring, or in any ornament, 

 can be seen with a magnifying-glass in their natural dimensions. 

 Adapted to the kind of magnifier already described fpage 235) as the 

 Stanhope Magnifier, these little objects carry with them the micro- 

 scope which enables them to be seen enlarged in every detail. A spot, 

 hardly perceptible to the naked eye, becomes a whole page of a book 

 which may be read as easily as the original. This charming inven- 

 tion we owe to M. Dagron. 



This is, however, a mere object of curiosity and fancy ; but micro- 

 scopic photography is not restricted to these miniatures of a doubtful 

 interest. It is applied to useful reproductions, and it has found a 

 wide scope in zoological and vegetable micrography. 



In presenting to the Academy of Sciences microscopic proofs of 

 diatoms obtained with different magnifying powers, M. Girard thus 

 expressed himself on the means he employed means, identical witli 

 those of ordinary photography, with the sole difference, that the 

 reproducing object-glass is replaced by a much 'smaller one, illumin- 

 ated by solar light reflected by means of a plane or concave mirror, 

 according to circumstances. A glass of a bluish shade is inter- 

 posed to absorb a portion of the non-actinic light. When it fails in 

 intensity, as when deep object-glasses are used whose front lens is 

 hardly a millimetre in diameter, it is necessary to have recourse to a 

 condenser. 



" Photomicography," says M. Girard, " is a perfectly exact means of 

 resolving the most difficult tests ; the image obtained proves, in a 

 manner not to be refuted, the value of the optic system of the micro- 

 scope. It enables us, further, to catch distinct effects of light, other- 

 wise unattainable ; interference and diffraction often give rise to 

 remarkable combinations." l 



The same author has made another application of microscopic 

 photography by studying, with the aid of polarized light, the crystals 

 of certain salts. 



1 Comptts Rehdus (1869). 



