88 OF THE SENSES OF 



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 elastic solid 

 body can be set in vibration, 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 etherial 

 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 in 

 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. 

 We may thus conceive the possibility of 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 vibra- 

 tions altogether beyond our limits, as Dr Wollaston 

 has ingeniously imagined, (we may almost say, 



