THE STEREOSCOPE AS A METHOD OF WORK- 

 ING OUT THE PRINCIPLES OF VISUAL 

 INTERPRETATION 



By Joseph Hershey Bair 



How we see is a subject, it seems to me, for various reasons, that 

 ought to evoke a greater popular interest. It is a problem that has elicited 

 the attention of the philosophers from the Greeks down. Before any 

 definite method was evolved by which visual facts could be observed 

 and formulated, and a consistent theory advanced, much speculation 

 regarding the factors involved in visual interpretation took place. In 

 the light of modern scientific knowledge many of the points of view 

 found in the history of philosophy on this subject seem ridiculous. 



One of the first writers on the right track toward positive knowledge 

 regarding vision was Descartes. x According to him both the accommoda- 

 tion of the refractive media of the eye and the convergence of the optic 

 axes contribute to perception of depth. Berkeley 2 was the first to attempt 

 to show how we perceive distance by sight. He pointed out that we 

 all know that when we look at an object, whether it approaches or recedes 

 from us, we alter the disposition of our eyes by lessening or widening 

 the distance between the pupils. This alteration of the eyes is attended 

 by sensations to which experience fixes a value for distance. The 

 mind finds constantly certain relations between strain sensations, or 

 confusion of the image, and the distance of the object; and there arise 

 in the mind connections between the various degrees of confusion, or 

 strain, and the distances which become habitual. 



In the development of definite knowledge of visual factors the stereo- 

 scope has played an important part. This instrument (see Figs. 5 and 

 9) was first described in 1834 by Professor Elliott of Edinburgh Univer- 

 sity, but was not made by him until 1839. In the meantime, 1838, 



1 Cartesius, Dioptrice, VI. 



a Berkeley, An Essay towards a New Theory of Vision (1709). 



175 



