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LIII. The use of Silver Films in improved Instruments of the 

 Camera-Lucida Class. By John 0. Douglas*. 



THESE instruments are divisible into two classes, viz. : — 

 that in which a reflected image of the object is seen 

 while the tracing-point is seen direct; and the other class, in 

 which the object or tracing is seen by reflection, but the tra- 

 cing-point or object is seen by light transmitted through a 

 plate which acts at the same time as a reflector. 



The forms in most common use are the camera lucida and 

 the steel disk or Soemmering' s mirror (of the first class), and 

 the parallel plate or tinted glass reflector (of the second class). 

 There are other forms, less common, but each referable to one 

 of the two classes described above. Instruments of the first 

 class give a brilliant and well-defined reflected image ; but 

 they are fatiguing to use, and some persons experience great 

 difficulty in using them. Instruments on the other principle 

 are far more easily used, they cause less fatigue ; but the re- 

 flected image is not so brilliant. In the case of the plane glass 

 reflector the definition cannot be so good, as both surfaces of 

 the glass reflect, and there are therefore two superimposed 

 images which do not exactly coincide ; the second reflection, 

 however, is weakened by using tinted glass ; and this colour- 

 ing also serves to reduce the transmitted light, which would 

 otherwise flood out the weak reflected image. 



What is required in an instrument of this kind is the bril- 

 liancy and clear definition of the camera lucida, combined 

 with the simplicity and ease in use and the cheapness of the 

 tinted plane glass reflector, with facility, where desired, for 

 using two reflections in order that the reflected image may not 

 be reversed. 



I believe these requirements are attainable by the use of 

 silver films on glass. Silver films are so highly reflective that 

 two or more successive reflections may be used if desired ; by 

 transmitted light the colour of the film is suitable for tinting 

 the glass. The thickness of the film may be regulated accord- 

 ing to requirements — a thick film being used when reflection 

 only is required, and a thinner one according to the ratio de- 

 sired between the reflected and transmitted light. The reflec- 

 tive power of the thinnest film is greatly superior to that of 

 glass. The silver film is applicable to most forms in use, and 

 it may be used not only on plane but on curved surfaces : e. g. 



* Communicated by the Author, having been read at the Meeting of 

 the Asiatic Society of Bengal, Calcutta, on the 7th of April, 1880. 



Phil. Mag. S. 5. Vol 9. No. 58. June 1880. 2 G 



