412 Mr. J. C. Douglas on the Use of Silver Films. 





■m 



represented in fig. 3. In Fig. 3. 



the instrument exhibited, 

 the larger reflector was 

 l£"xl|" and placed 10" 

 from the paper; the field 

 was about 4J in. square. 

 This instrument may be 

 used horizontal or in- 

 clined ; and it is admi- 

 rably adapted for draw- 

 ing such obj ects as insects, 



leaves, shells, &c. If the vertical distances between the mir- 

 rors and the object and paper respectively be constant, in in- 

 struments of this form the relative magnitudes of object and 

 drawing will obviously vary with the distance between the 

 reflectors. It is evident that by the use of reflectors in instru- 

 ments of this class, the reflecting surfaces may be larger and 

 the distance between them greater than if a prism were used. 

 The above are only examples of the application of silver films 

 to a particular class of instruments ; it is evident that they 

 offer great facility for giving this class of instruments its 

 maximum development. It is obvious also that silver films 

 are applicable with advantage in many other cases where 

 prisms are used at present, particularly where it is desired to 

 divide a beam of light into two ; e. g. if fig. 3 be turned up- 

 side down and the two eyes of the observer be in the place of 

 the arrow and the dotted line, the diagram represents an 

 arrangement suitable for a non-stereoscopic binocular micro- 

 scope, the inclination between the mirrors being varied to suit 

 the distance between the eyes. The loss of light in such an 

 arrangement would be very little ; and the brilliancy of the two 

 images might be rendered very nearly equal. To illustrate 

 how cheaply such instruments may be made, the mirrors in 

 the instruments exhibited were mounted in tubes of thin sheet 

 zinc, which is readily cut with ordinary scissors and bent into 

 shape with pliers ; a coat of asphalt varnish used for making 

 shallow cells was applied for the sake of appearance. 



The instrument, fig. 2, was fastened to the eyepiece by a 

 piece of zinc bent half round the eyepiece-tube, and held 

 against it by a small elastic rubber band. The tinted reflector 

 was supported by a bent plate of zinc, hung on the milled edge 

 of the eyepiece by a groove passing almost half round the 

 eyepiece : this is a most convenient method of attaching the 

 reflector or camera to the eyepiece, as it is quite firm enough, 

 and yet removable in an instant without disturbing the micro- 

 scope. 



