154 EXPERIMENTAL GENERAL SCIENCE 



eye, and the iris, like the opening in the shutter, can be enlarged 

 or diminished to admit the proper amount of light. In our 

 eyes, a continuous picture is produced as long as the eyes are 

 open, and no shutter to keep out the light is necessary, though 

 the eyelids may function like shutters on occasion. 



130. Persistence of Images. When an image is formed on 

 the retina, it does not vanish instantly, but persists for about 

 Ho second. A glowing stick whirled about on a dark night, 

 therefore, appears like a fiery circle since it is seen in several 

 positions in a short space of time and the images overlap as it 

 were. It is this peculiarity of the eye that makes moving 

 pictures possible. A series of pictures, each differing slightly 



FIG. 56. Formation of an image by a telescope, b-a is the real image; 

 d-c is the virtual image seen by the observer. (Tower, Smith and Turton.) 



from the one which precedes it, are thrown on the screen and 

 these, reflected to the retina, blend into a picture in which the 

 figures seem to move. The eye may be easily deceived in 

 other ways. For instance, a square ruled with perpendicular 

 lines appears to be shorter than one ruled with horizontal 

 lines. 



131. Various Effects of Light. Since light is a form of 

 energy, it is not surprising to note that it causes numerous 

 changes in matter Most of these effects are probably chem- 

 ical in nature, as the fading of colors, the tanning of the skin, 

 and the killing of germs. Hydrogen and chlorine mixed in the 

 dark are inert and do not change, but if a ray of sunlight strike 

 them, they combine with explosive violence. Another famil- 



