318 Scientific Notices. 



^' 5G7. Although any kind of impulse, or motions regulated by 

 any law, may be transferred from molecule to molecule in an elas* 

 tic medium, yet in the [undulatory] theory of light it is supposed 

 that only such primary impulses as recur according to regular pe- 

 riodical laws, at equal intervals of time, and repeated many times 

 in succession, can affect our organs with the sensation of light. 

 To put in motion the molecules of the nerves of our retina with 

 sufficient efficacy, it is necessary that the almost infinitely minute 

 impulse of the adjacent ethereal molecules should be often and 

 regularly repeated, so as to multiply, and, as it were, concentrate 

 their effect. Thus, as a great pendulum may be set in swing by 

 a very minute force often applied at intervals exactly equal to its 

 time of oscillation, or as one clastic solid body can be set in vibra- 

 tion by the vibration of another at a distance, propagated through 

 the air, if in exact unison, even so may we conceive the gross 

 fibres of the nerves of the retina to be thrown into motion by the 

 continual repetition of the ethereal pulses ; and such only will be 

 thus agitated, as from their size, shape, or elasticity are susceptible 

 of vibrating in times exactly equal to those at which the impulses 

 are repeated. Thus it is easy to conceive how the limits of visible 

 colour may be established ; for if there be no nervous fibres ia 

 unison with vibrations more or less frequent than certain limits, 

 such vibrations, though they reach the retina, will produce no 

 sensation. Thus, too, a single impulse, or an irregularly repeated 

 one, produces no light ; and thus also may the vibrations excited 

 in the retina continue a sensible time after the exciting cause has 

 ceased, prolonging the sensation of light (especially of a vivid one) 

 for an instant in the eye in the manner described, (Art. 543.) 

 We may thus conceive the possibility oj other animals, such as 

 insects, incapable of being affected with any of our colours, and 

 receiving their whole stock of luminous impressions from a class of 

 vibrations altogether beyond our limits, as Dr. Wollaston has in- 

 geniously imagined (we may almost say proved) to be the case 

 with their perceptions of sound." 



E. W. B. 



