$ Transactions of the Society. 



4. The Hygienic; Importance of Binocular Observation. 



We know that in most " Introductions " on the use of the 

 Microscope one is advised when working to change about from one 

 eye to the other, and we likewise know that it is the custom not 

 to follow this good advice ! On the contrary, most microscopists 

 have accustomed themselves to such an extent to the use of the 

 one eye, that if they have to use the other eye for any length of 

 time they experience acute discomfort. Very often they cannot 

 use it at all. When stopping work after hours of tiring observa- 

 tion, everyone has probably noticed that it is not the eye which 

 one has been using which is most tired, but the one which has 

 been out of service and which was apparently idle. Some observers 

 have even assured me that after working for a long period with 

 the right eye they have noticed a disturbance in the sight of the 

 left eye which has for some time hindered them when reading. 



An explanation of this fatigue of the unused eye, which, by 

 the way, is noticeable with any form of continuous observation 

 with one eye, might be sought in the fact that the unemployed eye, 

 in seeking a suitable point to focus on, causes the muscles or accom- 

 modation to be continually on the move, backwards and forwards 

 so to speak, thus sustaining much more fatigue than the other eye, 

 whose accommodation remains practically unchanged during the 

 whole period of observation. It may, however, be just as likely 

 that the seat of fatigue is more central, i.e. in the brain, for while 

 we are working with the Microscope we have to ignore entirely 

 the images received by one eye while concentrating our attention 

 on the images received by the other. The idle eye has to be 

 continually " called to order," i.e. it has to be forced into inactivity, 

 a process which absorbs a good deal of " energy." However, this 

 last-named discomfort only affects the beginner. With continued 

 practice the impressions received by the one eye are suppressed 

 unconsciously and without difficulty. It is no concern of the 

 science of optics to decide which of these or perhaps other explana- 

 tions is the correct one. 



Owing to this fatigue not only is the period of observation 

 reduced, but perhaps its value is diminished. At any rate Amann * 

 considers that it is not improbable that, owing to the constant 

 brain-effort which is required, the power and the discriminating 

 efficiency of the working eye might be injuriously affected. 



Asa matter of fact I found it possible to carry on observations 

 with the new binocular Microscope for a much longer period. It 

 is astonishing how comfortable and how little fatiguing it is. 

 Especially with dark-ground illumination the difference between 

 monocular and binocular observation is remarkably great. 



* Das Binokulare Mikroskop. Zeit. f. wiss. Mikr., xxvii. (1910) p. 492. 



