VISION IN INSECTS. 141 



vented from reaching- their internal extremity or 

 apex. The dipterous and neiu'opterous insects, 

 whose eyes contain thousands of facets and corre- 

 sponding" cones, are distinguished in general by their 

 more powerful sight from other insects ; and this is 

 owino; to the number of facets, &c., and not to the 

 size of the eyes ; for the size of the eyes merely in- 

 fluences 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, &c. 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 dif- 

 ferences 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 dilata- 

 tion, as at the time of our looking- at very near or 

 brightly illumined objects, or when we are in con- 

 siderable darkness with the pupil dilated perhaps to 

 its maximum, the smallest quantity of light will be 

 sufficient to enable us to distinguish 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 perceptibility of ouf 

 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 hght exists, the local di- 

 versities of clear, dark, and coloured parts in bodies 

 will hkewise be perceived, provided only those con- 

 ditions 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. 



