284 ACCESSORY APPARATUS 



and capable of being centred in its horizontal plane. (5) It should 

 be possible to easily separate the apparatus from the eye-piece and 

 replace it again in its former position at will. (6) The image of the 

 plane of the drawing, and the image of the microscopic object pro- 

 jected on it, must be seen with the apparatus without distortion. 

 As regards the first two conditions the arrangement of the original 

 Abbe camera is adopted, viz. two rectangular prisms with the hypo- 

 tenuses cemented together, of which one is silvered, with a small 

 portion of the silver deposit in the centre taken away, and with these 

 a second mirror A, fig. 222, for transmitting the image of the plane 

 of the drawing to this prism. But since one and the same prism, 

 with a fixed opening in its silver deposit, cannot suffice for all 

 purposes and changes of magnification, an arrangement is added by 

 which the prism P, fig. 223, with its fastening, can be easily taken 

 out of the apparatus and replaced by another \vith an opening of 

 different size. 



With respect to the third condition securing a due relation 

 between the intensities of the two images, an arrangement of two 

 smoked-glass wedges was made to move over each other so as 

 to form a plate of continuously varying thickness. This was most 

 satisfactory but too costly, so smoked-glass plates were employed 

 and set in the cylindrical wall of a small cap, R, figs. 222, 223, which 

 was simply placed over the prism. Each smoked glass in turn can 

 be interposed in the path of the rays by turning the cap on its 

 upper edge until a small pin engages in a corresponding small hole on 

 the lower edge of the cylinder. There are five smoked glasses of 

 different densities of colour, while one aperture is left empty. 



The adjustment in height is satisfied by the apparatus being 

 attached to the body-tube by means of a clamping screw, while the 

 adjustment from side to side is effected by the prism, together with 

 the cap and smoked-glass disc, being centred from front to back by 

 means of a screw, H, figs. 222, 223, working through a spring socket, 

 and from right to left by means of a second screw L, against which 

 works a counter-spring not shown in the figures. 



In order to pass conveniently from observation through this ap- 

 paratus to observation through the free eye-piece, the prism with its 

 diaphragm arrangement can be rotated to one side about a vertical 

 pin Z ; the return of the prism to its central position is marked by 

 a spring catch. To obtain drawings free from distortion, a drawing 

 table similar to that described by Dr. Bernhard ought to be 

 employed. 1 



This useful instrument has, however, been modified and made 

 simpler by more than one optical firm. Messrs. Swift have con- 

 structed a very handy and easily applied form, which is so arranged 

 that the microscope may be employed with it not only in the 

 vertical but also in an inclined position. It is illustrated in fig. 224. 



This camera lucida is precisely on the same principle as the Abbe 

 form used for the same purpose, but being manifestly less bulky it is 

 far more convenient and easier to use, although less efficient for very 

 careful work. 



1 Zeitschr. f. wiss. Mikr. xi. (1894), pp. 289-801. 



