] 96 Mr. S. Rowley on a New Theory of Vision. 



joining these points take other two points, A, B, distant re- 

 spectively from the point of bisection 6 and 18 inches. Draw 

 the straight lines CA, C'A, and perpendicular to them at A 

 the two lines EG, IK. Through the points F, F', taken 

 in the lines C A, C A, T V inch from C, C draw the lines F B, 

 F B, intersecting EG, I K, at b, b' . Through the points F and 

 d (a point in I K where a line passing through b perpendicular 

 to C C intersects it) draw a line. In this line, where a line 

 passing through B parallel to C C intersects it, take the point D. 

 The distance of this point from B will be 4>f 6 inches. 



Now suppose the points C, C to represent the centres of the 

 eyes; points F, F' the focal centres; A, B, D the situations of 

 luminous points ; the lines C A, C ; A the optic axes produced ; 

 the lines EG, IK lines of intersection where two surfaces slightly 

 convex toward the eyes, perpendicular respectively to the optic 

 axes at their point of meeting, cut the plane of the axes ; B F, 

 B F lines of luminous rays passing from the point B through 

 the points F, F'; D F' the line of a ray passing from D through 

 F' to the retina; b the point where the line BF intersects the 

 line E G ; and d the point of intersection of D F' with I K. 

 Then, if the above-enunciated proposition be true, to wit, that 

 an impression made by a luminous point on the retina of the 

 right or left eye becomes visible and is seen where a line drawn 

 from it through a point a little behind the centre of the crystal- 

 line lens pierces a slightly convex surface perpendicular to the 

 optic axis of that eye at the place of its meeting with the axis of 

 the other eye, the image of the point at B will be seen from the 

 left eye at b, and the image of the point at D from the right eye 

 at d, both in the same perpendicular of no variation*, or the 

 image of the point at B will be seen by the left eye a little be- 

 hind the place in which the image of the point at I) will be seen 

 by the right eye. 



The experiment may be made thus : — Take a thin rule (as a 

 plane scale) 12 inches in length with the right hand, and another 

 rule with the left hand. Hold the rule in the right hand with 

 its wide sides perpendicular to the plane of the horizon, in a line 

 running away from the middle point between the eyes so that 

 the near end shall be distant 6 inches, and the remote end rest 

 upon a horizontal bar of a well-lighted window at right angles to 

 the rule, and parallel to the eyes. 



To obtain the former distance with accuracy, it is necessary to 

 have two small objects, as two pins, placed equidistantly on op- 

 posite sides of the point of intersection of the bar by the rule, 

 distant from each other 4^ inches ; and when these shall be 



* By perpendicular of no variation is meant a line perpendicular to the 

 base of vision, for the definition of which see note, p. 203. 



