Art. XXVII.—Xote on the Binocular phenomena observed by 

 Professor Nipher • by Joseph LeConte. 



The striking phenomenon described by Prof. Nipher, in the 

 January number of this Journal, was described by Prof. W. B. 

 Eogers, at the Newport meeting of the American Assoc -ti.ui. 

 1860, and may be found recorded in the 14th vol. of Pr 

 p. 197. It is only one of innumerable phenomena which are 

 all explicable on the same general principles. The principles 

 involved have been so fully set forth in several of a 

 ou binoen, m iy ? "On a new method of 



representing visual phenomena," and VlV'On so-call. 

 of illusion,"* that it is unnecessary to do more than briefly 

 recall them here. 



_ 1. The field of view may be regarded as an outward projec- 

 tion of retinal states. As each eye has its own retina crowded 

 with its own retinal images, so also each eve must have its own 

 held of view and its own external images." There are therefore 

 for every object. These may be united, so 

 as to see the object single; or may be separated, so as to pro- 

 duce the phenomenon of double images. Moreover, the external 

 images of different objects may also be brought together and 

 superposed at will. In all discussions of binocular phenomena, 

 therefore, \\ _ ;UT t } lut we speak not of objects 



but of external images, the signs of objects. 



, 2 - In bi ith the optic axes parallel, as in 



looking at a distant object, the whole visual field wil 

 external images or objects is shifted by the right eye a half 

 interocular distance to the left and by the left eye the same 

 distance to the right ; so that the two visual lines are brought 

 together in the middle, and combine to form a common median 

 visual line, and the images of all objects in these lines are 

 bought together and superposed. This is the necessary result 

 ot the law of corresponding points. Retinal images of" all ob- 

 jects in the visual lines fall on corresponding points, viz: the 

 central spots; their external images are therefore thrown to the 

 same place in space and are therefore superposed. 



JNow in the experiment of Prof. Nipher, the visual line of 

 tne right eye passes through the axis of the tube and pierces 

 wi,;i Cer !l er ' k ' area of the object regarded, 



while the visual lme of the left eye pierces the left hand or 



abon f rt °U °f heT ° bjeCt Placed befo, - e '* * a P° in * distant 

 TWf ?£ (an mterocular s P a ce) from the axis of the tube. 

 eve ™h ^ ° enter ° f the Circular area re S arded b ^ the ri S ht 

 eye, and the spot on the hand or paper sheet pierced by the 



