The Binocular Microscope. By F. Jentzsch. 11 



6. The Summation of Stimuli in Binocular Vision. 



Although one generally has rather too much than too little 

 light in the Microscope, it is necessary that we should be quite 

 clear about the conditions of light intensity in the new instru- 

 ment, as one might easily expect a certain amount of obscuration. 

 First of all only about half the light transmitted by the objective 

 reaches each eye-piece.* Further, a certain percentage is absorbed 

 in its passage through the prisms and lost by reflection. 



Experience teaches us that if there be any obscuration at all 

 in the new binocular Microscope as compared with the ordinary 

 Microscope, this does not appear to be as great as calculation 

 would lead one to expect. This cpuestion of light intensity has to 

 be handled with a certain amount of caution. For when we have 

 arrived objectively at the determination of a certain degree of 

 illumination, we have still to deal, in the case of an optical instru- 

 ment, with its use considered subjectively, for here the sensibility 

 to light impressions has to be taken into account. 



It is known that in ordinary vision the same impression of 

 light intensity is obtained with two eyes as with one. One can 

 easily convince oneself of this by closing one eye when observing 

 an illuminated surface. If the proper precautions are taken no 

 obscuration will be observed. We know that in an experiment 

 of this kind the pupil of the open eye becomes dilated. It is not 

 possible, however, that this should simply make up for the loss 

 of light. For, owing to the comparative slowness of this reflex 

 action, a slight shadow should appear to cover the image during 

 the first moment. This, however, is not the case. The experi- 

 ment, moreover, only succeeds in a good light, and only then it 

 the object is so far distant that it can be observed easily and well 

 by both eyes, and provided that the person making the test is not 

 accustomed to observe with one eye only, which happens rather 

 often. 



The opposite result, namely that the apparent illumination of 

 a surface is greater when observed with two eyes than with one, 

 is generally arrived at if a diaphragm be interposed in such a way 

 that one eye sees only part of the test surface. On making the 

 fields of vision coincide, the part seen by both eyes appears brighter 

 than the other. According to this, therefore, binocular summation 

 of light stimuli obtains in everyday life. I do not, however, 

 consider this experiment to be decisive, for a fusion with the 



* The light absorbed by the silver coating may be entirely neglected. No 

 colouring of the images can be observed, although from theoretical considerations 

 this might be expected owing to dispersion from the silver. 



