172 ACTION OF LIGHT ON THE RETINA. SECT. XXI. 



sensation of sound or light. In order to excite vision, 

 the vibrations of the molecules of ether must be regular, 

 periodical, and very often repeated; and as the ear 

 continues to be agitated for a short time after the im- 

 pulse by which alone a sound becomes continuous, so 

 also the fibres of the retina, according to M. d'Arcet, 

 continue to vibrate for about the eighth part of a second, 

 after the exciting cause has ceased. Every one must 

 have observed, when a strong impression is made by a 

 bright light, that an object remains visible for a short 

 time after shutting the eyes, which is supposed to be 

 in consequence of the continued vibrations of the fibres 

 of the retina. Occasionally the retina becomes insen- 

 sible to feebly illuminated objects when continuously 

 presented. If the eye be turned aside for a moment, 

 the object becomes again visible. It is probably on this 

 account that the owl makes so peculiar a motion with 

 its head when looking at objects in the twilight. It is 

 quite possible that many vibrations may be excited in 

 the ethereal medium incapable of producing undulations 

 in the fibres of the human retina, which yet have a 

 powerful effect on those of other animals or of insects. 

 Such may receive luminous impressions of which wo 

 are totally unconscious, and at the same time they may 

 be insensible to the light and colors which affect our 

 eyes ; their perceptions beginning where ours end. 



SECTION XXL 



Polarization of Light Defined Polarization by Refraction Properties of 

 the Tourmaline Double Refraction All doubly Refracted Light is 

 Polarized Properties of Iceland Spar Tourmaline absorbs one of the 

 two Refracted Rays Undulations of Natural Light Undulations of 

 Polarized Light The Optic Axes of Crystals M. Fresnel's Discoveries 

 on the Rays passing along the Optic Axis Polarization by Reflection. 



IN giving a sketch of the constitution of light, it is 

 impossible to omit the extraordinary property of its po- 

 larization, "the phenomena of which," Sir John Her- 

 schel says, "are so singular and various, that to one 

 who has only studied the common branches of physical 

 optics it is liko entering into a new world, so splendid 



