106 FATIGUE FACTORS IN OPTICAL INSTRUMENTS 



I have put in as an exhibit a working model of a headpiece 

 attachable to any standard tube microscope without necessitating any 

 structural alteration in existing patterns, although it is intended to 

 arrange a bracket with attachment at or below the trunnion, in 

 any new pattern microscope available. Diagram and models illus- 

 trating application of this dyoptikon head-rest (eye-piece) to tele- 

 scopes, etc., are also shown. (Fig. 2.) 



I am also showing, by permission of Professor A. D. Waller, 

 F.R.S., his early ajid original negatives showing the electrical 

 response of retina to impacts of radiant energy of short duration; 

 also solution of so-called " Visual purple " which exists in colloidal 

 solution bathing the receptor organ (retinal cones and rods), a 

 photochemical substance which under the above circumstances of 

 external stimuli presents a balanced reversible reaction, and has been 

 shown to flow into the central fovea where no rods exist. Its 

 " sensitivity " is in accordance with Planck's minimum quantum of 

 energy, and has been shown by Victor Henri to be several thousand 

 times more sensitive to light than the most sensitive photographic 

 plate, on rapid exposure. Spectroscopically, " Visual purple " shows 

 no absorption bands. 



