VISION 



187 



2. Arrangement of Apparatus, (a) A convenient object to 

 observe is a well-illuminated window, or one sash of a window. 

 Measure the vertical distance between the horizontal strips of the 

 sash. 



(6) 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 distances 5m., 5.5 m., and 6 m. 



3. Operation. (1) Remove an eye from the rabbit which has 

 been chloroformed some time before and suspended by the anterior 

 limbs. 



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

 it, all of the areolar connective tissue, muscle tissue, etc., down to 

 the glistening, smooth sclera. 



(3) Wrap around its equator a band of absorbent cotton wet 

 with normal solution. 



FIG. 79 



Diagram of the dioptric system of the eye : R, point where visual line enters cornea ; P, prin- 

 cipal point of dioptric system ; N, nodal point ; o, object ; i, image ; distance o N and i N may 

 equal o and i, respectively. 



(4) Fix the eye in the clamp with its axis transverse to the axis 

 of the clamp, taking care to exert just enough pressure to prevent 

 the eye from falling on being touched, but not enough to distort it. 



(5) Fix to the clamp a thread with a bit of lead to serve as a 

 plumb line. 



4. Observations. (1) Adjust the support so that the eye is directed 

 toward the object and the image is located approximately symmetric- 

 ally about the fovea centralis and the plumb line over the mark 

 5m. With the fine dividers measure in the image the distance 

 between those points which were chosen as the limits of the object. 

 The value of this measurement may be read to tenths of millimetres 

 by laying the divider points upon the steel rule and reading with 

 the hand lens. 



