542 Intelligence and Miscellaneous Articles. 



I have made use of a simple plate of glass having a breadth of 

 1 decimetre and a length of 1*5. Semi-silvered glass has great 

 reflecting-power, and yet remains very transparent; it presents 

 merely a slight brown shade. 



It is known that M. Foucault advised the investing with this 

 semi-silvering the objectives of telescopes for viewing the sun, in 

 order to arrest nearly the whole of the rays of obscure heat*. 



When the glass is silvered, washed, and dried, the silver (which 

 might be removed by the slightest friction) is fixed by coating the 

 glass with a transparent varnish. For this purpose it is heated to 

 about 40°, and the following varnish is poured upon the silvered 

 face : — alcohol, 100 cubic centims. ; mastic tears, 10 grams. The 

 thin film of resin which adheres is very transparent and has a 

 very even surface. The reflecting-power of the glas£ is slightly 

 diminished, but is still sufficient. The silvered surface could be 

 covered with another glass plate ; but this would give rise to mul- 

 tiple reflection, which is avoided by using the varnish. 



The glass is then fixed, when the varnish is dry, by one of its 

 edges, in a nipper fitted to a foot, permitting various inclinations 

 to the horizontal to be given to the glass ; if the object to be drawn 

 is vertical, the angle of 45° should evidently be preferred. The 

 paper on which the drawing is to be made is fixed beneath. It is 

 indispensable that above the glass a sight-piece be placed, to give 

 the eye a perfectly fixed position. If the object has a strong 

 relief, the images of the various parts are formed at different dis- 

 tances behind the glass, and the perspective changes with the po- 

 sition of the eye ; it is the same with the coincidence of the points 

 on the paper and the different parts of the image to be drawn. The 

 sight-piece consists simply of a small slip of blackened cardboard 

 pierced with a small aperture ; this can be supported by the appa- 

 ratus which sustains the glass. 



If the illumination of the object, placed at a suitable height and 

 distance before the inclined glass, be in a certain correspondence 

 with that of the drawing-paper, the image of the object, the pencil, 

 and even the line of the drawing as it is being executed can all be 

 seen at the same time without any fatigue. The conditions of the 

 illumination can be easily realized by the aid of screens or shutters. 



The advantage of this camera lucida over that generally used 

 arises from the reflection taking place over a large surface, which 

 gives more intensity, and especially from the circumstance that the 

 simultaneous visibility of the pencil and the image is independent 

 of the position of the eye of the observer, depending only on cer- 

 tain conditions of illumination which can be easily regulated before 

 commencing the execution of the drawing. It would be easy, by 

 taking two parallel glass plates, one semi-silvered and the other 

 having received a thick coat of silver, to make a camera lucida that 



* The same arrangement would be very advantageously employed in 

 photographic enlarging-apparatus, where the solar heat sometimes cracks 

 the plates. 



