330 M. F. August on a new Species 



survive the period of the eyes' vibration (if the expression may 

 be allowed) ; and thus we have the durable representation of the 

 entire body, whose dimensions we estimate by the difference of 

 the convergence of corresponding rays. To this theory Dove 

 has objected that the two images of a stereoscope may be seen 

 united so as to give the impression of solidity by the almost in- 

 stantaneous illumination of the electrio spark : this, however, 

 may be explained on the grounds of an incredibly rapid move- 

 ment of the eye, however improbable that may be. Briicke's 

 theory has therefore continued tq be generally received without 

 any essential modification, even on account of Meissner's* pro- 

 found research concerning the horopter and the position of the 

 corresponding retina«-points, or of the controversy connected 

 therewith. 



2. The experiment above-mentioned is as follows : — A thin 

 cylinder of metal, as straight and smoothly polished as possible, 

 is so united with a fixed axis that its own geometrical axis cuts the 

 other at right angles. The experiment may be performed with 

 sufficient accuracy with two needles fixed in a cork at right angles 

 to each other. The one needle forms the rotating cylinder, the 

 other the axis of rotation. If the cylinder be placed at rest in 

 the sunlight and regarded on either side with one eye, then, pro- 

 vided its length be sufficient, the reflexion of the sun, or rather 

 a bright spot (which we shall call the point of reflexion), will be 

 observed somewhere in its length. If now the first eye be shut 

 and the cylinder be regarded with the other, the point of reflexion 

 will be perceived in another position. On opening both eyes, 

 two points will be seen at the same time in different places ; and 

 if lines be imagined drawn from each of these points to the corre- 

 sponding eye (which lines may be called the reflected rays " corre- 

 sponding to each eye"), it is obvious that these lines do not lie 

 in the same plane except when the cylinder itself lies in the same 

 plane with the line joining the optic centres of both eyes, which 

 we shall call as usual the " ground-line." If the cylinder be 

 rotated, each of the points of reflexion will obviously change its 

 place on the cylinder, and will therefore describe a curve in the 

 plane of rotation; and this curve, if the rotation be sufficiently 

 rapid, will appear continuously illuminated. A somewhat dif- 

 ferent curve will of course appear when the apparatus is regarded 

 with the other eye. If, however, the apparatus be regarded with 

 both eyes, instead of two curves in the plane of rotation as might 

 be expected, a single curve of double curvature is generally per- 

 ceived, not confined to the plane of rotation, but formed by the 

 intersection of the conical surfaces described by the two reflected 

 rays during rotation. 



* Beitr'dge zur Physiologie des Sehorgans. Leipzig, 1854. 



