214 LAB OR A TORY G UIDE IN PH YSIOL OGY. 



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 cot- 

 ton wet with normal solution. 



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

 the axis of the clamp, t?king 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 

 symmetrically about the fovea centralis, and the plumb 

 line over the mark 4 meters. 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 millimeters by laying the divider points upon the 

 steel rule and reading with the hand lens. 



(2) Make similar observations at 4.5 m., 5 m., 5.5 m., 

 and 6 m. Each observation should be made three or 

 four times and the average taken. 



(o) Record these averages in a table ruled with columns 

 for the values d, o, i, n and /. 



(4) Calculate for column n the values obtained by sub- 

 stituting, in the formula n = ^, the values observed in 

 (1) and (2). What is the value of n ? 



(5) Measure the antero-posterior diameter of the eye. 

 How far anterior to the posterior surface of the sclera 

 is n located? How far from the surface of the cornea? 

 How does the ratio of these two quantities differ from 

 that given above for the human eye? 



(6) Locate the position of the principal point or the 



