410 Mr. J. C. Douglas on the use of Silver Films in 



a plane concave lens silvered on the plane side might be used 

 by a shorted-sighted person instead of the common plane re- 

 flector used in sketching microscopic objects ; a slight curva- 

 ture of the first or second reflecting surface in the camera 

 lucida might be used to render it unnecessary to employ a lens 

 to equalize the sensibly different distances of the images of the 

 object and plane of delineation. The cost of silver films on 

 glass is very trifling ; and if taken care of they last for years. 

 A number might be made at intervals ; or they might be sup- 

 plied for a trifling sum by the opticians. 



For many purposes the films might be deposited on thin 

 glass and varnished, or protected by glass, when they would 

 be very durable and would bear handling. For some purposes 

 the film might be thickened by electro-deposition and removed 

 from the glass. As the films are so cheap, a number of gra- 

 duated thicknesses might be kept, and a suitable one selected 

 in each case to adjust the relative brilliancies of the reflected 

 and transmitted light ; or the films might be applied as the 

 dark glasses usually supplied with the camera lucida : but this 

 seems less simple and convenient than the use of a thicker or 

 thinner film as transmitting reflector. A silver surface may 

 reflect upwards of 90 per cent, of the incident light ; a total 

 reflecting prism has been found to reflect only about 75 per 

 cent, or less, the loss being due to reflection at the first sur- 

 face and absorption ; the superiority of the silver surface is 

 evident, particularly where several successive reflections are 

 required. Even if the highest attainable brilliancy be not 

 generally required, still the higher this is the greater the range 

 of adjustment without alteration of the source of light. The 

 strictest regularity in the film not being essential, suitable 

 films are very readily obtained. With strict cleanliness, pure 

 chemicals, care that the glass is wetted equally in every part 

 by water or alcohol at the moment of immersion in the silver- 

 ing solution, and care that the solution is properly mixed (i. e. 

 homogeneous), success is readily attained. 



I find* that Professor Govi, of Rome, has devised a form 

 of camera lucida in which a metallic film is used. He simply 

 gilds the reflecting surface of the camera-lucida prism with a 

 thin film of gold, and cements to this surface with Canada 

 balsam another similar prism. M. Nachet has adopted this 

 improvement in the construction of various forms of camera 

 lucida. 



The greater advantages of the silver film are obvious. By 

 the use of silvered glass, instruments of various forms and 

 of large size may be readily constructed for a trifling sum 

 * Annual Kecord of Science and Industry, 1875, p, 144. 



