LEPIDOPTEROUS INSECTS. 87 



those that are near, from the contraction of their 

 eyes. The experiments of Dr Evans and Sir G. S. 

 Mackenzie, both tend to support Wildman's views. 



We are yet but imperfectly acquainted with the 

 vision of insects ; and, from the great variety in the 

 construction of their eyes, it is no wonder we should 

 be so. For example, a centipede has twenty eyes, a 

 spider has eight, and a butterfly and its congeners but 

 two ; but these two have thirty-five thousand facets 

 in each. It may, therefore, seem remarkable, how 

 they see but one object ; but it is not a more difficult 

 question, than how we see but one object with two 

 eyes. 



When the facetted eye of a butterfly is examined 

 a little closely, it will be found to have the appear- 

 ance of a multiplying glass, the sides, or facets, 

 nearly resembling a brilliant cut diamond. 



In the experiments performed by Mr Herschel, he 

 describes the impulses received by the eyes of insects 

 as analogous to those of sound, as given by Wollaston. 

 He says, " Although any kind of impulse or 

 motions, regulated by any law, may be transferred 

 from a molecule in an elastic medium ; yet, in 

 the undulating theory of light, it is supposed that 

 only such primary impulses as occur according to 

 regular periodical 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 etherial molecule* 



