VtSlON IN INSECTS. 141 



vented from reaching their internal extremity or apex. 

 The dipterous and neuropterous insects, whose eyes 

 contain thousands of facets and corresponding cones, 

 are distinguished in general by their more powerful 

 sight from other insects ; and this is owing to the num- 

 ber of facets, Sec, and not to the size of the eyes ; for 

 the size of the eyes merely influences the extent of 

 their visual horizon. 



' From this statement, it may be inferred that the 

 vision of the compound eyes must be very imperfect 

 and indistinct ; but at the same time, no doubt, it is 

 amply sufficient for the wants of insects, Slc. The 

 quantity of light which enters into the interior of the 

 eye is also very small ; but the optic nerve is proba- 

 bly so constituted as to perceive the faintest differ- 

 ences in the intensity of light and colours. Of the 

 whole light emitted or reflected by exterior objects, we 

 ourselves receive into the eye only that portion which 

 the pupil is capable of admitting ; and yet, when the 

 pupil is at its minimum of dilatation, as at the time of 

 our looking at very near or brightly illumined objects, 

 or when we are in considerable darkness with the 

 pupil dilated perhaps to its maximum, the smallest 

 quantity of light will be sufficient to enable us to dis- 

 tinguish the general forms of bodies. A light of 

 moderate intensity, with a mean degree of dilatation of 

 the pupil, seems best suited to the degree of percep- 

 tibility of our sense of vision ; for, when the pupil is 

 widely dilated, as by means of belladonna, objects at 

 other times moderately bright then become dazzling. 

 As soon as the general sensation of light ex- 

 ists, the local diversities of clear, dark, and coloured 

 parts in bodies will hkewise be perceived, provided 

 only those conditions are present which are required 

 for the proper insulation of the different kinds of 

 rays.'* 



* Loudon's Mag. of Nat. Hist., vol. iv, p. 124, &c. 



