132 INSECT MlSCELtiANIBlS. 



Mr. Herschel seems inclined to make observa* 

 tions, with regard to the vision of insects, somewhat 

 analogous to those of Dr. Wollaston in the in- 

 stance of hearing. It may not be improper to give 

 his own statement of his views. *' Although," 

 says he, " any kind of impulse, or motions, re- 

 gulated by any law, may be transferred from a mole- 

 cule in an elastic medium, yet in the undulating 

 theory of light, it is supposed that only such primary 

 impulses as recur according to regular periodical laws 

 at equal intervals of time, and repeated many times 

 in succession, can atfect our organs with the sensa- 

 tion of light. To put in motion the molecules of the 

 nerves of our retina with sufficient efficacy, it is ne- 

 cessary 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 eifect. Thus, as a great pendulum 

 may be set in swing by a very minute force often ap- 

 plied, at intervals exactly equal to its time of oscilla- 

 tion, 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 

 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 con- 

 ceive how the limits of visible colour may be esta- 

 blished ; 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 im- 

 pulse, or an irregularly repeated one, produces no 

 light ; and thus, also, may the vibrations excited in 

 the retina continvie a sensible time after the exciting 



