VISION 189 



line of direct vision before the eye, one point being about 25 cm. 

 distant from the eye and the other at arm's-length ; make the obser- 

 vations with one eye, the other being closed or screened. 



(a) Focus upon the near point. Is the image of the distant point 

 clear? 



(6) Focus upon the distant point. Is the image of the near point 

 clear? 



(c) While the eye is steadily focused upon the near point bring the 

 distant point slowly up to a position beside the near point. One of 

 the images is transformed from an illy defined one to a clearly defined 

 one. Which image is it? Does one note a similar change in the 

 definition of the image when he moves the near point out to a posi- 

 tion beside the distant point while focusing steadily at the latter? 



(d) Sum up the results of the experiments into a concisely formu- 

 lated statement. 



(2) Holding the points side by side at a distance of 20 cm., note that 

 the points appear equally well defined. 



(a) Direct the eye steadily at one of the points while moving the 

 other nearer to the eye. Note the number of centimetres which it 

 advances toward the eye before the outlines become illy defined. 

 Reverse the act, moving the point back to its original position beside 

 the stationary point, noting that the image of the receding point 

 remains clear. 



(6) Continue to carry it farther from the eye, noting that after it 

 has been carried beyond the unmoved focused point a certain dis- 

 tance the outline becomes illy defined. Note the number of centi- 

 metres between the two points in this position. 



(c) Make a similar experiment, using 30 cm. for the distance of 

 the stationary point, and note the centimetres between the points at 

 the limits of clear definition. In this way one may observe and 

 measure the focal depth of the eye. 



(d) Is the focal depth greater at 20 cm. or at 30 cm. ? 



(e) Tabulate the focal depths of the members of the class for the 

 distances 20 cm., 30 cm., 40 cm., 50 cm., and 60 cm. 



(/) Sum up the results of the experiment into a concisely formu- 

 lated statement, and show the relation between ocular focal depth 

 and microscopic focal depth. 



(3) Determination of the Near Point or " Function Proximum." Deter- 

 mine the distance from the eye of the nearest point at which a pencil 

 point or needle may be perfectly clearly seen. The exact location of 

 the near point may be more satisfactorily determined if one look at 

 the object through two pinholes, 2 mm. apart, in a card. At this 

 point the punctum proximum the act of accommodation is brought 

 actively into play. 



(4) Determination of the Punctum Remotum. (a) Direct the eye 

 toward some object not less than 6 m. away and describe to the 



