ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 819 



The light can fall either on the left or in front. Now turn the ring 

 of the prism to the left, lift the vertical pin sufficiently so that the 

 prism can be placed in a horizontal position over the eye-piece, and 

 push the horizontal pin in so far that the circular opening of the 

 prism is over the centre of the eye-piece. Now place behind the 

 Microscope a di'awing-board, oi an atlas with the hinge or back 

 touching the foot of the instrument, or about 2 inches away from it, 

 whose upper cover can, by some contrivance placed within it, be 

 inclined about 18°. On this the drawing-paper is to be laid. If the 

 prism has been placed with the horizontal surface so far round the 

 vertical pin backwards that the clear circle visible in the eye-piece, 

 on looking through it from a vertical direction, is about halved by 

 the sharp edge of the prism, we see through the half of this circle 

 which is visible through the prism, not only the object clearly, but also 

 the paper with the pencil. If the further margin of the field of view 

 appears somewhat less clearly on the paper, lift the anterior part of 

 the prism by tui*ning it on the horizontal pin until it is clear, where- 

 upon by depressing the vertical pin the prism is placed as near the 

 eye-piece as possible. In order, however, that both images shall be 

 equally clear it is necessary that both should be equally illuminated. 

 If the pencil-point appears faint and dim, either the paper must be 

 more strongly illuminated by changing the position of the whole 

 apparatus, or the object must be darkened by altering the mirror, 

 and vice versa. Short-sighted people must use a lens when the 

 distance of the pencil is greater than that of their natural vision." 



Silver Films for Instruments of the Camera Lucida Class.* — 

 Mr. J. C. Douglas, Secretary and Treasui'er of the Asiatic Society of 

 Bengal, in a paper on this subject, points out that what is required in 

 an instrument of this kind is the brilliancy 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 the facility when 

 desired for using two reflections, in order that the reflected image may 

 not be reversed, and he believes these requirements are attainable by 

 the use of silver films on glass. 



" These are so highly reflective that two or more successive reflec- 

 tions 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 according to requirements, a thick film being used 

 when reflection only is required, and a thinner one according to the 

 ratio desired between the reflected and transmitted light. The 

 reflective power of the thinnest film is greatly superior to that of glass. 

 The silver film is applicable to most foi-ms in use, and it may be used 

 not only on plane but on curved surfaces, e. g. a plane concave lens, 

 silvered on the plane side, might be used by a short-sighted person 

 instead of the common plane reflector used in sketching microscopical 

 objects, a slight curvature of the first or second reflecting sui'face in 

 the camera lucida might be used to render it unnecessary to employ a 

 lens to equalize the sensibly difi'erent distances of the images of the 



* Proc. Asiat. Soc. Bengal, 1880, pp. 73-6. 



