Accessory Apparatus. 



A. Drawing Apparatus. 



The drawing surface in all drawing apparatus of our make is rendered 

 visible by double reflection, while the pencils of rays emerging from the micro- 

 scopes or magnifiers reach the eye by a direct path. 



In addition to the old form of "Camera hicida" - the drawing prism. 

 No. 109, shown in Fig. 33 and introduced by us in 1869 the drawing apparatus 

 designed by Professor ABBE has been supplied by us for about the last 20 years. 



For a special Drawing Apparatus for low maf/nificationx see 



pp. 67 69. 



While in the drawing prism, as also in most other existing forms of drawing 

 apparatus, only a semi-circular portion of the exit pupil can be utilised, in the 

 ABBE drawing^ camera the full area of the pencil is effective. Loss of light 

 is therefore avoided even when using the highest magnification, 

 while both the pencil and the drawing surface remain quite clearly visible. This 

 material advantage is gained by the application of a small cube, composed 

 of two equal rectangular prisms cemented together -- the "ABBE cube" 

 in the position of the eye-point of the ocular. The cemented surface of the 

 upper prism is silvered but has a central aperture of 1 or 2 mm diameter. 



The brightness of the drawing surface and of the image can be modified 



in all our drawing apparatus by the interposition of smoked glasses. With 

 drawing prism, No. 109, as also with the small ABBE drawing ap- 

 paratus, No. 110, one or two smoked glasses may be inserted into the prism 

 casing. The .brightness of the image may be moderated in either of two ways, 

 i. e., by appropriate stopping or by the use of smoked glasses in connection 

 with the illuminating source. 



For the purpose indicated above the large ABBE drawing apparatus, 

 Nos. Ill and 112, are fitted with a cap encasing the prisms from above, which 

 is provided with moderating glasses of varying degrees of 

 illumination, and with a similarly equipped eccentric disc pivoted 

 below the prisms. In both appliances only five of the six openings contain 

 smoked glass, the sixth remaining clear so that the cap and disc may remain 



