366 Discoveries of MUller and others in the 



and continuous image is produced. Rays coming from one 

 point of a remote object will, it is true, illuminate throughout 

 more than a single cone ; and then, to each luminous point 

 without, there will correspond in the interior of the eye, not 

 exactly a single illuminated point, but rather a little circle 

 of diffused or dispersed light; and, in consequence, an 

 image of but little distinctness will be reproduced on the 

 sentient surface or retina ; the distinctness of the image of 

 course increasing in proportion as the object approaches the 

 eye. 



The image in the interior of the eye will be more distinct, 

 precisely as the cones, in a given portion of the eye, are more 

 numerous ; the distinctness will also increase in proportion 

 to the length of the cones; for the longer the cones are, 

 the more completely will all rays entering them obliquely be 

 prevented from reaching their internal extremity or apex. 

 The dipterous and neuropterous insects, whose eyes contain 

 thousands of facets and corresponding cones, are distin- 

 guished in general by their more powerful sight from other 

 insects ; and this is owing to the number of facets, &c., and 

 not to the size of the eyes; for the size of the eye 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, &c. The quantity of light which enters into the 

 interior of the eye is also very small ; but the optic nerve is 

 probably so constituted as to perceive the faintest differences 

 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 di- 

 latation, 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 distinguish the ge- 

 neral 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 our sense of vision ; for, when 

 the pupil is widely dilated, as by means of belladonna, 

 objects at other times moderately bright then become daz- 

 zling. As soon as the general sensation of light exists, the 

 local diversities of clear, dark, and coloured parts in bodies 

 will likewise be perceived, provided only those conditions are 

 present which are required for the proper insulation of the 

 different kinds of rays. 



