OF CERTAIN OPTICAL INSTRUMENTS 



105 



In order to avoid mal-orientation and resulting fatigue, it is 

 necessary to provide the observer with a working bench of adequate 

 height, correlated height and 'position of chair, suited to the physique 

 of each observer. 



Fig. 2. 



Dyoptikon (Pivotal) Head-Rest 



(Universal Letters Patent). 



Scale -One-Half. 



