The Binocular Microscope. By F. Jcntzsch. 9 



5. The Superiority of Binocular Observation. 



The appearance of the microscopic image differs qualitatively 

 from that with the monocular type of instrument. The first 

 consideration is that in binocular observation one generally sees 

 better than otherwise, and moreover one is able to detect more 

 detail. It must, however, be admitted that there are marked 

 differences in this respect in individual cases. The fact that more 

 detailed examination is possible leads one to think that possibly 

 an actual increase in visual power takes place in binocular work. 

 Certain experiments tend to support this theory, but I have tried 

 to explain the effect in the following manner. 



According to the Duplex vision theory of von Kries, we possess 

 two entirely different methods of vision, i.e. " daylight vision " and 

 "twilight vision." Now the sensitive surface of the retina consists 

 of two different sets of organs, the rods and cones, of which the 

 former receive principally colour impressions while the latter are 

 acted on by differences of light intensity. According to the above 

 mentioned Duplex vision theory the cones are the organs for 

 daylight vision and the rods for seeing in the semi-darkness. 



Now it is often pointed out that the rods are missing from the 

 macula lutca, therefore with direct vision only the colour-dis- 

 criminating cones are employed and the rods only play a part in 

 indirect vision or possibly only in the twilight. This is not stating 

 the case quite correctly. The rods do not disappear entirely in 

 the region of direct vision. They are not absent from the whole 

 area centralis* but only from the innermost spot, the fovea 

 centralis, that is a region which externally only covers a field of 

 vision of about 1° to 1'5°. Round about this region the rods 

 make their appearance without any marked boundary line and 

 varying greatly in individual cases. They increase in number 

 farther away from the centre while the number of cones decreases. 

 Besides these there are certain qualitative differences to be noted. 

 At the points where the rods commence to diminish the cones 

 gradually assume the shape of the former. This similarity is 

 most marked in the fovea centralis itself. 



During normal vision (excepting perhaps with very high light- 

 intensities) the cones and rods act at the same time ; only that the 

 rods have a greater capacity for adapting themselves to darkness, 

 so that a weak intensity of light suffices to excite the action of the 

 rods but is insufficient to affect the cones. In the same way when 

 one is using the Microscope both these organs are generally active. 



In addition to the differences in intensity of light we have 



* This expression is considered to be more suitable than the synonymous 

 macula lutea, as according to Gullstrand the yellow colour of the so-called yellow 

 region only refers to a post-mortem effect. 



