Proceedings of the British Association. 159 



observations. He also communicated a supplement to the former 

 Report of the Committee on Waves. He divided waves into 

 three great orders, obeying different laws. 1. Wave of the first 

 order, — the wave of translation, — solitary, progressive, depending 

 chiefly on the depth of the fluid : of two species, positive and 

 negative. 2. Waves of the second order, — oscillatory waves, — 

 gregarious : the time of oscillation depending on the amplitude 

 of the wave. 3. Waves of the third order : capillary waves, — 

 gregarious. The oscillations of the superficial film of a fluid, 

 under the influence of the capillary forces, extending to a very 

 minute depth : short in duration : of two species, — free and con- 

 strained. The various phenomena of these classes of waves had 

 been thoroughly examined by the committee, and many inter- 

 esting results developed. 



On a very curious fact connected with Photography, discovered 

 by M. Moser, of Konigsberg* communicated by Prof. Bessel to 

 Sir D. Brewster. The following are the general facts connected 

 with the discovery. A black plate of horn or agate is placed be- 

 low a polished surface of silver at the distance of one twentieth 

 of an inch, and remains there for ten minutes. The surface of 

 the silver receives an impression of the figure, writing or crest, 

 which may be cut upon the agate or horn. The figures do not 

 appear on the silver at the expiration of the ten minutes, but are 

 rendered visible by exposing the silver plate to vapor, either of 

 amber, water, mercury, or any other fluid. He (Sir D. Brewster) 

 had heard Prof. Bessel say, that the vapors of different fluids 

 were analogous to the different colored rays of the spectrum ; 

 that the different fluids had different effects, corresponding to 

 those of the spectrum ; and that they could, in consequence of 

 such correspondence, produce a red, blue, or violet color. The 

 image of the camera obscura might be projected on any surface 

 —glass, silver, or the smooth leather cover of a book — without 

 any previous preparation ; and the effects would be the same as 

 those produced on a silver plate covered with iodine. 



This paper gave rise to an animated conversation, in the course 

 of which M. Bessel said that he had seen some of the pictures 

 taken by this process, which were nearly, but not quite, as good 



* See Dr. Draper's letter of reclamation of this discovery, at page 203 of this 

 number. 



