10 Transactions of the Society. 



above all things to observe fine colour differences or shades. Now 

 as both eyes are rarely equally efficient, it may happen that one 

 eye is more fitted for the one, while the other eye is particularly 

 well equipped to do the other kind of work. Thus if we are in a 

 position to use both eyes we can also utilize their respective 

 strongest points. 



Everyone who makes much use of binocular instruments is 

 aware that the two eyes assist each other to a much greater extent 

 than would be expected from general consideration. With this 

 method of observation there is not only a, continual movement of 

 accommodation, as is the case in monocular instrument working, in 

 virtue of which as is known the penetrative efficiency of the 

 instrument is increased, but the attention, the perceptive organs of 

 the main cerebral centres, turn from one eye to the other as it 

 were, thus combining the delicate shades of colour as seen by one 

 eye with the fine contour lines of the image as seen by the other. 



The process as described need not take place actually in such 

 a simple manner. The capacity of our sense of sight is not ex- 

 hausted by the mere perceptions of colour and light intensity. In 

 analysing the sense of sight one would rather bracket the light- and 

 colour-sense together as one factor and would then add the optical 

 sense of space and position-perceptions, and finally the capacity 

 for optical resolution and the sense of form. Although with the 

 ordinary phenomena of sight all these senses come into action 

 simultaneously, there will, nevertheless, generally speaking, always 

 be differences between the two eyes of any individual with re- 

 spect to these different phases. There may also be differences in 

 the degree of sensitiveness of corresponding points on the retina. 

 It may here be mentioned that the unpractised eye is generally 

 less sharpsighted than the practised eye, while having greater 

 sensitiveness to light. 



All these differences are naturally less noticeable with the 

 binocular method of observation than with the monocular, so that 

 we can now more easily realize how it is that one can make 

 better observations with a binocular than with the ordinary 

 Microscope. Moreover, this conclusion holds not only for the 

 Microscope but also for many measurements made by the aid of 

 optical instruments, especially in photometric work. Observa- 

 tions in all these cases are directly comparable with normal 

 binocular observations on far distances. 



It is common experience that long distance observations made 

 from an isolated mountain peak or from a balloon are rendered 

 much more effective by using both eyes. It will also be necessary 

 to take into consideration the binocular summation of stimuli and 

 the so-called " vividity " of perception. 



