200 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



the ciliary body. Give the lens used to concentrate the light a gentle 

 rocking motion. The visual field will then appear to the subject as 

 reddish yellow, through which dark figures are passing. The latter 

 assume the character of a network, in agreement with the branching 

 blood-vessels (Purkinje's figures). By this means shadows of the 

 blood-vessels are cast upon regions of the retina not ordinarily exposed 

 to them. It is essential, however, that these shadows move, because 

 the retina quickly adapts itself to continued stimulation. 



Make a pinhole in a card and hold it directly in front of one eye, the 

 other being closed. Direct the visual axis upon a bright and evenly 

 illuminated sheet of white paper placed in front of a lamp. Move the 

 card from side to side. Vertically running figures will be apparent. 

 Move the card up and down. Horizontal vessels will now be in evidence. 



Gaze at a white cloud through a thick piece of blue glass. Many 

 bright points followed by shadows will be seen to traverse the visual 

 field in a constant procession. The latter, in all probability, represent 

 the red blood-corpuscles. 



5. Direct and Indirect Vision. Draw a figure upon a white sheet 

 of paper, consisting of one central dot surrounded by six others at a 

 slight distance from it. With your right eye fixate a small object at 

 a distance of 50 cm. from you. Bring the aforesaid figure slowly into 

 the line of vision. Note that the individual dots coalesce at some 

 distance from the latter. They are severally in evidence only when the 

 figure is brought into the visual line. Obviously, at this time, the rays 

 of light emitted by the figure are focalized in the fovea, i. e., in the 

 region of the greatest acuity of vision. When T ,<made to fall upon out- 

 lying districts of the retina the rays cannot form a precise image. 



Gaze at an object in a brightly illuminated room. Note that the 

 object is unconsciously brought into direct line with the fovea. Darken 

 the room. Note that at first your vision is very poor, but gradually 

 adapts itself to the low intensity of light. Also observe that you now 

 endeavor to focalize objects by bringing them upon the outer zones of 

 the retinae. 



Focalize a single word upon a page of print. Note that only the 

 word looked at is perfectly clear, while those farther away from it in 

 any direction are not clear. 



Draw radiate lines upon the blackboard and equip each with cross 

 marks at distances of 5 degrees from one another. Fix any one of the 

 cross marks and note that the others are at this time not perfectly clear. 



6. Associated Movements of the Eyes. Close one eye and place 

 the tip of your index-finger upon the upper eyelid. Note that the focal- 

 izing of an object with the other eye invariably produces a co-ordinated 

 movement in the closed eye. 



7. Positive After-images. Having rested the eyes for a short time, 

 look suddenly at a lighted incandescent lamp and shut the eyes again 

 after a distinct image of the lamp has been formed. Note the after- 

 effect. 



