178 OF SIGHT. 



exceed, according to Jurin, that of each, by more than 

 one thirteenth part. 



It is needless to add, what the celebrated painter, 

 Leonardo da Vinci, long since remarked, — that in 

 viewing distant objects, it is preferable to employ but 

 one eye.*(F) 



281. Sight can never occur unless the angle of vision 

 exceeds 34 seconds. This was proved by the very beau- 

 tiful experiments k of the acute Tob. Mayer, who for- 

 merly was one of our number. And he demonstrated 

 the great excellence of the human sight, by shewing 

 that this still remained the limit of vision under any 

 light, — under the splendor of the meridian sun and the 

 faint light of a lantern ; so that vision remains almost 

 equally clear although the light be considerably di- 

 minished, f 



282. We may hence infer the prodigious minuteness 

 of the images of objects projected upon the retina, J 

 and nevertheless impressed so forcibly upon it, that, 

 under certain circumstances, their vestiges remain, after 

 the removal of the objects from before the eye. § 



• Consult Lambert, sur la partie photomttrique de tart da peintre in the 

 Mem. de f.4cad. des Sciences de Berlin. 17 (18. p. 80 sq. 



f Tob. Mayer, Experimenta circa vims aciem, in the Commentar. Soc. 

 Scient. Gottingen. T. iv. 



J De la Hire, Reddens de la vue. p. 375. 



§ Gaxsendi, Vita Peireskii. p. 175 sq. Hague. 1655. 4to. 



Franklin, letters on Philosophical Subjects, at the end of his Expts. on Elec- 

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



ltob. War. Darwin, Experimenta nora de spectris s. imaginibus ocularibus, 

 qua ot'jrctis Ittcidioribus antea rius, in oculo dauso vel aver so percipiuntur. 

 Lue.l.Bat. 1785. 4to. 



Er. Darwin, Zuonomia. T. i. 



C. H.mly, Dtblioth. Ophthalmolog. T. i. P. ii. p. i. 



