The Binocular Microscope. By F. Jentzsch. 13 



concert to which we listen only inattentively is the example of an 

 intense but not vivid perception. 



The difference seems to be related to the difference between 

 attentive and inattentive observation, although it is not identical 

 with it. For the greater impressiveness of a perception under 

 equal objective intensity may, apart from the question of attentive- 

 ness, be conditioned by a multiplication of the areas susceptible to 

 stimuli. The orchestra does not become less loud if we hear it 

 through only one ear, nevertheless we feel the desire to increase its 

 vividity by listening with both ears. Also, we do not always see 

 more intently with two eyes than with one, but more vividly. 



I am convinced that this holds good for all kinds of binocular 

 instruments : thus the advantage of prismatic binocular field-glasses 

 over the so-called " prismatic monoculars " lies not merely in their 

 stereoscopic effect, which is in any case only apparent with com- 

 paratively near objects, but especially in the vividity, that is, the 

 general increase in the " lifelikeness " of the impression, which is 

 brought about by binocular as against monocular vision. In the 

 new Microscope this advantage is similarly noticeable. 



Now I will go a little further, and should like to make the 

 suggestion that in the impression of vividity is included also a 

 part of the sense of depth, that is, those psychological factors 

 which occur only in binocular vision. For the sense of depth 

 (spatial effect) is known to be not only a function of the impres- 

 sions on the senses, but is composed of actual optical factors and 

 of physiological and psychological effects. If you remove by any 

 method the immediate perception of depth by submitting to the 

 two eyes two identical images, the remaining physiological and 

 psychological factors can still produce a conception of depth (spatial 

 effect). 



The estimation of the distance of a thing takes place according 

 to the size of objects of a known extension : one judges from the 

 appearance of the perspectives (covering, cast-shadows, intensity of 

 colours, so called " air-perspectives ") and many other incidental 

 facts. Further physiological features may be mentioned, such as, 

 the straining of accommodation and the convergence of the two 

 visual axes. 



In the new instrument not only are the purely optical con- 

 ditions for depth- effect absent (the two images are identical) but 

 the physiological factors are also eliminated (both eyes are parallel 

 and are accommodated to infinity). The psychological effects 

 which accompany an impression of depth may, however, be pro- 

 duced by some conditions and so give rise to a certain depth- 

 effect. The majority of these accessory impulses in connexion 

 with the sense of depth are also to be considered in connexion 

 with monocular vision : some, however, only appear with binocular 

 vision. Thus, for many observers, the stimulus for the conception 



