TALBOT'S LAW. 277 



12. Inversion of Shadows thrown on the Retina. 



(a.) Make three pin-holes in a card, and arrange them 

 in a triangle close to each other. Hold the card four or five 

 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. 



(a.) 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 sensa- 

 tion. The intensity of the 

 light impression is quite inde- 

 pendent of the absolute dura- 

 tion of the periods of illumi- 

 nation and shade. 



(a.) Rotate a disc like 

 Fig. 128 twenty-five 

 times per second, then 

 the period in which 

 illumination and shade 

 alternately lasts for the 

 inner zone is -^ sec., for 

 the middle 5^, and for the Fig. 128. 



outer zone T ^ 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. 



