354 



SCIENCE. 



[N. S. Vol. VII. No. 167. 



teresting experiments on ' Binocular Factors in 

 Monocular Vision. ' This title is somewhat mis- 

 leading. The essential characteristic of binoc- 

 ular vision consists in the simultaneous forma- 

 tion of slightly dissimilar images on the two 

 retinas, with corresponding modification of the 

 perception of depth in space. Mr. Judd's ex- 

 periments relate to variation in direction of the 

 two visual lines, with resulting production of 

 double images ; but fusion of these images is an 

 indispensable requisite for the attainment of 

 any binocular perception. 



It is well known that most persons fail to 

 perceive double images as phenomena attendant 

 upon binocular vision. To perform binocular 

 experiments the observer must have some train- 

 ing in the muscular control of the eyes, and 

 also in visual perception. Such experiments 

 occupied much of my attention some years ago 

 {American Journal of Science, 1881-1883). In 

 performing the first experiment described by 

 Mr. Judd it is very easy to catch the heterony- 

 mous image, and by proper control of the eye 

 to stop its motion instantly. The appearance 

 of unrest of the object, to which he refers, is 

 due to the motion of this image during the in- 

 stant before fusion is attained. The visual line 

 of the closed eye, as Mr. Judd correctly ob- 

 serves, does not converge toward that of the 

 open eye. Since fusion of images is attained in 

 natural binocular vision and without any con- 

 scious effort, on suddenly opening the unused 

 eye, unconscious motion of both eyes results 

 until fusion is secured. But the vision is strictly 

 monocular until such fusion is completed, and 

 the momentary illusion is not a binocular factor 

 in such vision. 



The experiment is perhaps most easily ac- 

 complished by covering one eye with the hand 

 and suddenly removing this, instead of bring- 

 ing the muscles of the eyelids into play. If the 

 open eye be directed to some well illuminated 

 object of known diameter and at a known dis- 

 tance, such as a clock dial, the angular displace- 

 ment of the heteronymous image is easily found. 

 It is only necessary to control the unused eye, 

 resisting the tendency to secure fusion and not- 

 ing the interval between corresponding edges 

 of the two overlapping images. The ratio of 

 this to the distance gives the angle. 



The unused eye will in most cases be so 

 directed that the two visual lines are approxi- 

 mately parallel. In cases of strabismus, exter- 

 nal or internal, this parallelism is, of course, 

 lost, but in such cases there is usually no power 

 of binocular perception, one eye being habitu- 

 ally depended upon to the exclusion of the 

 other. 



When control of the eyes is lost temporarily 

 through drowsiness the uncontrolled relation 

 of the visual lines may be ascertained by wink- 

 ing one eye, if the observer is enough interested 

 in binocular experiments to remember this, and 

 to do this, in his semi-conscious condition. I 

 have done so repeatedly, and have always found 

 in my own case that the double images were 

 homonymous ; which indicates that the visual 

 lines were crossed instead of divergent. I have 

 watched the eyes of others under such conditions. 

 In some cases the contraction of the rectus 

 muscles was seen to be internal, in others ex- 

 ternal. No general rule on this subject can 

 be formulated. It seems highly probable, how- 

 ever, that after consciousness becomes com- 

 plete all the rectus muscles are completely 

 relaxed, with more or less divergence of visual 

 lines. By the aid of stereographs upon which 

 the stereographic interval exceeds the obser- 

 ver's interocular distance, binocular vision by 

 optic divergence is readily attained after a 

 •reasonable degree of muscular control of the 

 eyes has been attained by practice. But for 

 obvious reasons the external rectus muscles are 

 comparatively but little under the control of 

 the will, and 7° or 8° of such divergence is 

 probably about a maximum for normal eyes. 



I have elsewhere shown {Am. Jour. Sci., 

 May, 1882) that the ciliary muscle is also sub- 

 ject to the control of the will, though its action 

 is most generally automatic. My observation 

 accords with that of Mr. Judd that vision with a 

 single eye is rarely if ever equal in distinctness 

 to that with two eyes. But the accommodation 

 of the single eye improves with time. 



W. Le Conte Stevens. 

 Rensselaer Polytechnic Institute, 

 Troy, N. Y. 



the northern dtjrchmusterung. 

 The Durchmusterung charts of the northern 



