86 



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 lucida" - the drawing prism, 

 No. 109, shown in Fig. 46 and introduced by us in 1869 -- the drawing camera 

 designed by Professor A8BE has been supplied by us for about the last 20 years. 



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

 apparatus, only a semi-circular portion of the emerging pencil can be utilized, 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 so-called "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. 



By a simple arrangement facilities are given for centring the opening in 

 the silvered surface in relation to the emerging pupil of the microscope in such 

 a manner that the pencil of rays, issuing after double reflection from the drawing 

 surface and thence through the eye-piece, reaches the eye of the observer con- 

 centrically. 



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 camera, 

 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 appro- 

 priate stopping or by the use of smoked glasses in connection with the illumi- 

 nating source. 



