568 SIGHT. 



sight, by showing that this still remained the limit of vision in 

 any light, in the splendour of the meridian sun and the faint 

 light of a lantern ; so that vision remains almost equally distinct, 

 although the light be considerably diminished. d 



" We may hence infer the extreme minuteness of the images 

 of objects projected upon the retina 6 , and nevertheless impressed 

 so forcibly upon it, that, under certain circumstances, their ves- 

 tiges remain after the removal of the objects from before the 

 eye. f 



It has frequently been a question among philosophers, why 

 objects are seen erect, when the images which suggest them to 

 the mind are thus inverted. Some have supposed that infants at 

 first see objects upside down, and afterwards learn to correct 

 their erroneous sensation by comparing the information acquired 

 by touch with that obtained by sight. This opinion, held by Locke, 

 Lecat, Diderot, Buffon, &c., as well as that of our originally seeing 

 objects double and all as at the same distance and correcting these 

 errors by experience and the sense of touch, was amply refuted by 

 Bishop Berkley, and subsequently by Gall s and others. The law 

 of visible direction affords the true explanation. The simple state- 

 ment of this law is that each point of an object is seen in a line 

 perpendicular to the point of the retina on which its image fails. 

 The surface of the retina being concave and nearly as possible 

 spherical, these lines of visible direction meet and cross at a point 

 within the eye which is called the centre of visible direction : the 

 lines from the upper part of the image go to the lower part of 

 the object, and those from the lower part of the image proceed 

 to the upper part of the object. An inverted image thus neces- 

 sarily produces an erect object, and the external object is the 



* " "Fob. Mayer, Experimenta circa visits aciem, in the Commentar. Soc. Scient. 

 Getting, t. iv." 



e De la Hire, Acddens de la Vue, p. 375. " 



f " Gassendi, Vita Peireskii, p. 175. sq. Hague, 1655. 4to. 



Franklin, Letters on Pliilosophical Subjects, at the end of his Expts. on Elec- 

 tricity. Lond. 1769. 4to. p. 469. sq. 



Rob. War. Darwin, Experimenta nova de spectris s. imaginibus ocularibus, qua 

 objectis lucidioribus antea visis, in oculo clauso vel averse percipiuntur. Lugd. Bat. 

 1785. 4to. 



Dr. Darwin, Zoonomia, t.i. 

 ' C. Himly, Siblioth. Ophthalmolog. t. i. P. ii. p. 1." 



6 1. c. 4to. p. 180. sqq. 



