282 ACCESSOEY APPARATUS 



the determining- of the magnifying power of objectives. It is manifest 

 that the distance between the paper and the eye of the observer 

 cannot be so readily determined in this case as in those forms of the 

 instrument where the image of the paper and pencil is seen direct. 



The same apparatus arranged so that the prism casing together 

 with the mirror may be swung back while the clamping collar 

 remains on the tube in its adjusted position, is shown in fig. 221. 

 The mirror has a surface of 75x50 mm. (3x2 in.), and may be 

 inclined at any angle between the horizontal plane and 45, the 

 latter position being marked by a stop. The length of the arm 

 supporting the mirror being lO5cm. (4 in.), it is only with very 

 large drawings necessary to incline or raise the drawing surface. 



But the latest modification of this instrument is shown in figs. 

 222 and 223, where it will be observed that the camera is attached 

 to the tube by means of the clamping-ring K, and the Abbe double 



FIG. 221. Abbe's camera, improved. 



prism is centred by means of the screws L and H. The brightness 

 of the drawing surface and the microscopic image is respectively 

 regulated by a cap R encasing the prisms, which is provided with a 

 clear opening and five moderating glasses of varying degrees of 

 density, and by an eccentric disc B pivoted below the prisms, which 

 is also provided with a clear opening and five moderating glasses. 



In order to completely utilise the increased cone of emerging- 

 rays obtained with low magnifications, the usual prism, having in its 

 silvering an aperture of 1 mm., can quickly and conveniently be 

 exchanged for another with an aperture of 2 mm. 



The prism, together with the moderating glasses, may be turned 

 aside about the vertical pin Z into the position indicated by the 

 dotted lines shown in fig. 222. When the prism is returned to its 

 original position it is fixed by a catch, which is not externally 

 visible. 



In the use of a good drawing apparatus (1) the light from the 



