178 REFRACTION AND REFLECTION. SECT. XX. 



receive a motion which would cause the refracted ray, and the 

 part of the motion retained by the large ones would occasion the 

 reflected wave ; so that, when light passes through a plate of 

 glass or of any other medium differing in density from the air, 

 there is a reflection at both surfaces ; but this difference exists 

 between the two reflections, that one is caused by a vibration in 

 the same direction with that of the incident ray, and the other by 

 a vibration in the opposite direction. 



A single wave of air or ether would not produce the 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 impulse by which alone a sound becomes continu- 

 ous, 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. The interval of time during which 

 the impression lasts is longer for the blue than for red or white 

 light : it must not be less than 0"*34. 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 insensible to feebly illuminated objects when continu- 

 ously 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 vibra- 

 tions may be excited in the ethereal medium incapable of pro- 

 ducing 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 we are totally 

 unconscious, and at the same time they may be insensible to the 

 light and colours which affect our eyes, their perceptions begin- 

 ning where ours end. 



