104 FATIGUE FACTORS INCIDENTAL IN THE USE 



If an observer sits in the " auditorium " below the level of the 

 centre of the pictorial field of action, he is soon fatigued, and brow- 

 ache and discomfort ensue, for observation requires him to extend 

 his head slightly from the " primary position," to raise his eyelids 

 and rotate his eyes upwards. 



If now he reseats himself at a higher level (" dress circle " angle), 

 these factors disappear, for now his head is in the " primary 

 position," or slightly flexed, a position assumed by gravitation and 

 requiring very little muscular effort for its maintenance. 



It has been suggested that the lower seats in a bioscope theatre 

 should be tilted backwards with head rests, so that the necessary 



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Fig. 1. 



Diagram to indicate that the periphery of the retina is relatively less irrit- 

 able to impact of radiant energ-y causing- sensation of RED light, compared 

 with the FOVEA (direct vision), observing that the Ada-ptation -phase [Dark 

 or Light adapted) of the Retina does not affect the Red threshold. The 

 orange-yellow and blue (A600-400) require the same number of energy 

 anits at their threshold of Perception, and are affected by the phase of 

 adaptation of the Retina. 



slightly extended position of the head may be attained without 

 muscular effort and so avoid mal-orientafion of the head and eyes. 

 However, the promiscuous use of a head-rest in a place of public 

 resort is not a feasible or pleasant proposition. 



Fatigue factors, in connection with the observer's posture in 

 using the microscope, are also present, and for him no facility is 

 provided for resting his head and neck muscles, and insufficient atten- 

 tion is paid to the angle formed between the ocular and the vertical 

 plane of the observer's head. 



