VISION. 213 



clamps; meter stick or tape; steel or ivory rule, with 

 millimeters subdivided if possible, hand lens, fine divid- 

 ers with needle points; bone forceps; NaCl 0.6%; camel's 

 hair pencil; absorbent cotton. 



2. Preparation. (1) Mathematical. (See Fig. 29 B.) 

 We wish first to locate the nodal point in a rabbit's eye. 

 Represent the distance from the retina to the nodal 

 point by n, the distance from the object to the image by 

 d, the vertical dimension of the object by o, the same 

 dimension of the image by i. From the similar right 

 triangles of the figure one may write: 



(1) o: i = d n: n; 



(2) on = id in; 



*=& 



Jnder the conditions of the experiment i is so small 

 compared with o that it may be ignored in the denomi- 

 nator, and we may use the equation: 



(2) Arrangement of Apparatus. 



(a) A convenient object to observe is a well-illumi- 

 nated window, or one sash of a window; measure 

 the vertical distance between the horizontal strips 

 of the sash. 



() Arrange three or four tables end to end in a line 

 perpendicular to the plane of a window. On the 

 table lay off from the plane of the window the dis- 

 tances 4, 4.5, 5, 5.5 and 6 meters. 

 . Operation. 



(1) Remove an eye from the rabbit which had been 

 chloroformed some time before and suspended by the 

 anterior limbs. 



(2) Dissect from the eye, especially from the posterior 



