820 SUMMABY OF CURRENT RESEARCHES RELATING TO 



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 supplied 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 graduated 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 

 surface and absorption ; the superiority of the silver surface is evident, 

 particularly when several successive reflections are required. [We 

 understand the fact to be quite the converse, and that the advantage 

 is in favour of the prism. — Ed.] Even if the highest attainable bril- 

 liancy 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 silvering solution, and care that 

 the solution is properly mixed, i. e. homogeneous, success is readily 

 obtained. 



" Professor Govi, of Rome, 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 by any 

 ingenious person." 



The following is a description of the instruments exhibited at the 

 meeting of the Asiatic Society when the paper was read : — 



" 1. An ordinary tinted glass reflector for use with the Microscope. 

 The tinted glass usually used was replaced by a piece of glass covered 

 with a thin film of silver. The silvered side is turned towards the 

 eye-piece, and reflects the magnified image. In this form, several 

 reflectors, differing in the thickness of the silver film, should be avail- 

 able for regulating the ratio between the transmitted and reflected 

 light, but a certain thickness of film will be found which is applicable 

 to most purposes, so that change of reflector is seldom necessary. 



* See ' Annual Record of Science and Industry,' 1875, p. 144. 



