312 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



furnished, the result will be more than two million five hundred thou- 

 sand despatches produced during the two worst months of the year. 



" The pellicles were rolled into a quill which was tied by agents of 

 the administration to the pigeon's tail. Their extreme suppleness and 

 complete imperviousness adapted them for this service. My dry prepar- 

 ation has, besides the triple advantage : of being got ready in one ope- 

 ration, of having a uniform surface, and of not separating from the glass 

 on the appearance of the image ; it is worked with perfect security 

 and it is not exposed to the mischances of the ordinary processes." 



FIG. 223. Enlarging and reading the microscopic despatches during the siege of Paris. 



When the .microscopic despatches had reached Paris, they were 

 submitted to the operation of enlarging, and projected, by a solar 

 microscope, illuminated by the electric light, on a white board. There 

 a copy could be taken of their contents. The transparency of the 

 collodion films facilitated this projection, and the text was read with 

 ease. This was assuredly one of the most useful and ingenious 

 services which physical science and the art of photography could 

 furnish, though unfortunately too late, towards the national defence. 



