438 SUMMARY OF CURRENT RESEARCHES RELATING TO 



at all remarkable tLat different persons should vary in the interpreta- 

 tion they put upon sensations produced under the same external con- 

 ditions, although the general eifect of controlling the condition of the 

 eye among them may be much the same. The author has elsewhere 

 detailed numerous experiments on this subject.* The result may be 

 briefly stated by saying that, while the visual angle remains constant, 

 an increase in the contraction of the ciliary muscle, or of the internal 

 rectus muscle if both eyes be employed, produces the illusion that the 

 object is much smaller and nearer. Under such conditions, the apparent 

 diminution in size, together with imperfect focalization, may produce 

 as a secondary eftect the illusion that the visual angle has been dimi- 

 nished, and the imagination that the object is more distant. Thus 

 the unmistakable illusion is that of diminution of size, and this may 

 be coupled with great lack of determination in the judgment of 

 distance. Upon the author the most usual effect is that of diminution 

 of distance. 



The internal rectus and ciliary muscles are supplied from the same 

 nerve, and their contractions are usually simultaneous, though dis- 

 association to a limited extent is by no means impossible. The 

 relaxation of these muscles, with contraction of the external rectus, 

 produces the illusion of greater distance and size for the object 

 retinally pictured. This is in accordance with the laws of associa- 

 tion ; for, under ordinary circumstances, near vision requires con- 

 traction, and distant vision relaxation, of internal rectus and ciliary 

 muscles ; while unusual contraction of the external rectus muscles is 

 not unfrequently necessary in the ordinary use of the stereoscope, 

 involving discomfort and an illusion of increased distance in the 

 binocular picture. 



All our judgments, whether visual or otherwise, become vitiated 

 when the conditions are very different from those to which we are most 

 accustomed. Prof. Brewer's 440 observers accommodated their eyes, 

 as nearly as possible, to the same external conditions. The striking 

 diversity in the conclusions reached by them shows how various were 

 the muscular conditions under which they interpreted their own 

 sensations. To this must be added the important fact to which 

 attention was called by him, that for the same eye much depends upon 

 education. The mechanic who thought the picture looked to be 5 feet 

 long, and projected upon a screen, was quite unaccustomed to forming 

 judgments with no actual objects for comparison ; and in any event 

 there was, doubtless, room for improvement in his visional education. 



Another striking example of variation in judgment by the same 

 person, under changed ophthalmic conditions, is found in early expe- 

 riences with the binocular Microscope by the original inventor of this 

 instrument, Prof. J. L. Eiddell, of New Orleans, La. In looking 

 with both eyes at an object 10 in. distant, the two visual lines form 

 an angle of a little over 14'^, and a corresj)onding degree of con- 

 traction of the internal rectus muscles is necessitated. The two tubes 

 of Dr. Riddell's first binocular Microscope were sensibly parallel, the 

 sheaf of rays after passing through the objective being divided, and 



* Amer, Journ. Sci., November and December 1881, April and May 1882, 



