12 'Transactions of the Society. 



usually less bright image of the diaphragm itself takes place on 

 the apparently darkened portion of the field of vision. 



According to modern physiology, the binocular summation 

 of stimuli only takes place with the eye adapted to obscurity, 

 and in the case of vision in full light it is entirely absent. It 

 appears to me, however, that there are certain transition stages, 

 and that a very high degree of full light adaptation must indeed 

 be necessary to entirely eliminate any summation of stimuli. It 

 may even be discovered that the conditions of " twilight vision " 

 are already entered upon at much higher intensities than has 

 hitherto been supposed. I do not wish to go any further into 

 this question in all its bearings, but only wish to emphasize that, 

 according to my personal experience, this is actually the state of 

 the case with the new binocular Microscope. 



We know that a summation of stimuli takes place within one 

 eye when the object is very small, and when the image approaches 

 the size of an optical sensory element. In this case the intensity 

 of light is proportional in the first instance to the number of 

 sensory elements covered, but it will not increase as soon as the 

 stimulated surface has reached a certain size. I suppose, there- 

 fore, that also in binocular vision an analogous summation of 

 stimuli takes place (even with adaptation to bright light), as the 

 objects to be examined are very small. This would explain the 

 fact that on using both eyes in the new binocular Microscope a 

 marked increase in the impression of light intensity is noticeable. 

 However, it is also possible that this feature is greatly due to 

 another phenomenon generally experienced, the so-called "vividity." 



7. Vividity. 



In using the new instrument, one has another observation to 

 make which is not so easy to put into words. Perhaps the best 

 way of giving expression to it is to say that everything appears 

 more alive, more life-like than otherwise, so that the term 

 " vividity " is perhaps the most suitable. 



The expression " vividity " was introduced in psychological 

 terminology by Richard Semon in order to characterize the vivacity 

 of perception. Vividity is a quality of perception clearly dif- 

 ferent from " intensity " but not completely independent from 

 it. For we are able to perceive an object of very small intensity, 

 such as a distant light on a dark night, with great vividness 

 (vividity), and on the other hand the effect which a brilliant arc 

 lamp has may be one of very slight penetration. We hear, for 

 instance, the steps of a person carefully coming nearer on tiptoes 

 with great vivacity and distinctness though as something quite 

 noiseless. On the other hand, the fortissimo of a noisy open-air 



