342 PRACTICAL PHYSIOLOGY. [LXXII. 



One sees such floating objects as are present in the media of one's eye, the 

 "muscae volitantes." 



12. Inversion of Shadows thrown on the Retina. 



Make three pin-holes in a card, and arrange them in a triangle close to 

 each other. Hold the card 4 or 5 inches from the right eye, and look 

 through the holes at a bright sky or lamp. Close the left eye, and in front of 

 the right hold a pin so that it just touches the eyelashes. An inverted image 

 of the pin will be seen in each pin-hole. Retinal images, as we have seen, are 

 inverted on the retina, shadows on the retina are erect, and therefore the 

 latter, on being projected outwards into space, are seen inverted. 



13. Duration of Impressions. 



On a circular white disc, about half-way between the centre and 

 circumference, fix a small black oblong disc, and rapidly rotate it 

 by means of a rotating wheel. There appears a ring of grey on 

 the black, showing that the impression on the retina lasts a certain 

 time. 



14. Talbot's Law. A grey once produced is not changed by increased 

 rapidity of rotation of the disc exciting the sensation. The intensity of the 

 light impression is quite independent of the absolute duration of the periods 

 of illumination and shade. 



Rotate a disc like fig. 259 twenty-five times per second, then the period 

 in which illumination and shade alternately lasts for the inner zone is T V sec -> 

 for the middle J^-, and for the outer zone T ^ 7 sec. In all three zones the period 



of illumination lasts exactly one-half 

 of the period, and the three zones 

 , have exactly the same brightness. 

 Rotate more quickly, and no further 

 effect is produced. The number of 

 rotations is readily determined by 

 Harding's improved counter. 



15. Charpentier's Experi- 

 ments (slow-moving discs). 



(i.) " Black -band Experiment" 

 Make a disc J white, cause it 

 to revolve (once in two seconds) 

 in bright direct sunshine. On 

 the white sector will be seen a 

 FIG. 259. narrow " black band " or sector 



near the black edge that has 



just passed in front of the eye, but separated from that edge by 

 a narrow white sector (fig. 260). The black band always appears 

 at the same time from the moment the white sector appears in the 

 field. The time is equal to /^ to -^ second, i.e., 0.014" to 0.016". 

 It is independent of the velocity of the disc. Sometimes there 

 are two or three successive fainter bands, but they are difficult to 

 make out. 



