THE PERCEPTION OF SPACE. 223 



the other ; and within two similar quadrants, al and ar for 

 example, there should, if the correspondence were consist- 

 ently carried out, be geometrically similar points which, if 

 impressed at the same time by light emitted from the same 

 object, should cause that object to appear in the same direc- 

 tion to either eye. Experiment verifies this surmise. If 

 we look at the starry vault with parallel eyes, the stars all 

 seem single ; and the laws of perspective show that under 

 the circumstances the parallel light-rays coming from each 

 «tar must impinge on points within either retina which are 

 geometrically similar to each other. The same result may 

 be more artificially obtained. If we take two exactly simi- 

 lar pictures, smaller, or at least no larger, than those on an 

 ordinary stereoscopic slide, and if we look at them as 

 stereoscopic slides are looked at, that is, at one wdth each 

 eye (a median partition confining the view of either eye to 

 the picture opposite it), we shall see but one flat picture, 

 all of whose parts ajjpear sharp and single.* Identical 

 points being impressed, both ej^es see their object in the 

 same direction, and the two objects consequently coalesce 

 into one. 



The same thing may be shown in still another way. 

 With fixed head converge the eyes upon some conspicuous 

 objective point behind a pane of glass ; then close either 

 eye alternately and make a little ink-mark on the glass, 

 * covering ' the object as seen by the eye which is momen- 

 tarily open. On looking now with both eyes the ink-marks 

 wall seem single, and in the same direction as the objective 

 point. Conversely, let the eyes converge on a single ink- 



* Just so, a pair of spectacles held an inch or so from the eyes seem 

 like one large median glass. The faculty of seeing stereoscopic slides single 

 "R'ithout an instrument is of the utmost utility to the student of physio- 

 ktgicul optics, and persons with strong eyes can easily acquire it. The 

 ■only difficulty lies in dissociating the degree of accommodation from the 

 degree of convergence which it usually accompanies. If the right picture 

 is focussed by the right eye, the left by the left eye, the optic axes must 

 either be parallel or converge upon an imaginary point some distance 

 behind the plane of the pictures, according to the size and distance apart 

 of the pictures. The accommodation, however, has to be made for the 

 plane of the pictures itself, and a near accommodation with a far-off con- 

 vergence is something which the ordinary use of our eyes never teaches ua 

 to effect. 



