Schmidt & Haensch, Berlin S. 



89 



c) 1. Penumbra Apparatus, with adjustment for equal bright- 

 ness of the two halves of the field; for tubes of 200 mm length. 

 X 



Fig. 28. 



2. for tubes of 400 mm length, 



3. 000 .. 



Equipment and accessories similar to those of a. (Fig. 28.) 



d) The same apparatus with double], wedge compensation. 

 Original design. 



n 



Fig. 29. 



1. for tubes of 200 mm length, 



2- 400 



3. 600 

 Equipment and accessories similar to those of a. (Fig. 29.) 



Adjustment for equal colouring of the field is impossible in cases of 

 colour blindness and it is inexact where the capacity for discerning colours is 

 imperfect; besides, it is, with continued working, for the majority of eyes much 

 more straining than the adjustment for equal brightness. For this reason 

 the majority of analysts obtain more exact results with the penumbra 



