358 WELLS'S NATURAL PHILOSOPHY. 



evident by the fact that when a portion of the liglit is cut off hj a colore(3 

 glass, or a thin cloud, the image of tlie sua is seen distinctly. On the con- 

 trary, we fail to perceive many stars at night, because the images they pro- 

 duce on the retina are too faintly illuminated to produce sensation. That 

 some light from such stars actually enters the eye, is proved by the fact that 

 if we place a lens before the eye, and collect a greater quantity of their light 

 upon the retina, they at once become visible. 



Can the eye The Gje posscsscs a limited power of accom- 

 Ses'onuu- modating itself to various degrees of illumi- 

 ttiaation? natioD. Ill tliG dark, the pupil of the eye 

 enlarges its opening, and allows a greater number of rays 

 to fall upon the retina ; in the light, the pupil contracts 

 in proportion to the intensity of the illumination, and 

 diminishes the number of rays falling upon the retina. 



.yy^ . . This change does not take place instantaneously. "When 



from the light We leave a brilliantly illuminated apartment at night and go 

 do°we ^find'^u '^^^'^ *^^® ^^^^ Street, wo are unable for a few moments to seo 

 difficult at first any thing distinctly. The reason of this is, that the pupil of 

 thing? ^°^ ^^° "^y^i which has become contracted in the light, is unablo 

 to collect sufficient raj-s from the objects in the dark to see 

 them distinctly. In a few moments, however, the pupil dilates, allows moro 

 rays to pass througli its aperture, and we seo more distinctly. The reverse 

 of this takes place when wo go from the dark into the light. Cats, owls, and 

 some other animals are able to see distinctly in the dark, because they have 

 the power of enlarging the pupils of theur eyes so aa to collect the scattered 

 rays of light. 



Every impression made by light remains for a certain length of time on 

 the retina of the eye, according to the intensity of its efl'ects, and a measur- 

 able period is necessary to produce a sensation. 



.^j^^j. . "Wo are unablo, when riding rapidly on a railroad, to count 



prove the con- the posts of an adjoining fence, because the light from each 



imager on the ^°^^ ^^^^^ "P^"^ *'^° ^y^ ^^ ^^^^^ ^^P^'^ succession, that the dif- 

 retina after the ferent images become confused and blended, and wo do not 

 •ppeared ? '*'' obtain a distinct vision of the particular parts. 



If wo rotate a stick, lighted at ono end, somewhat rapidly, 

 it seems to produce a complete circle of fire ; the reason of this is, that tho 

 eye retains the image of any bright object for some little time after the object 

 is withdrawn ; and as the light of the stick returns to each particular point of 

 its path before the image previously formed has faded from the retina, it seems 

 to form a complete circle of fire. 



w^ . . This continuance of the impression of external objects on 



dark when we the retina after the light proceeding from them has ceased to 

 ^'" act, is tho reason also why w© are not sensiblo of darknesa 



when wo wink. 



