174 OF SIGHT. 



of objects, inverse indeed necessarily from the laws 

 of light. 



270. The focus which, in this mode, falls upon the 

 retina, is considered acute, not absolutely but rela- 

 tively, on account of the different refrangibility of 

 colours; but the latitude arising from this aberration 

 of the rays is so small that it not only does not obscure 

 the clearness of vision in any perceptible degree, but is 

 the source of many advantages.* 



271. The celebrated question — why we behold ob- 

 jects erect, while their image is painted inversely upon 

 the retina,f may be easily answered, by considering 

 that objects are called inverse relatively only to those 

 which appear erect. 



Now, since the images of all objects and of our own 

 bodies are painted on the retina, each in its relative 

 situation, this relative situation must correspond as 

 exactly as if they were viewed erect, so that the mind 

 (to which a sensation excited by the image and not the 

 image itself is communicated) is preserved from all 

 danger of error. 



272. Since many conditions are required for distinct 

 vision, the Creator has wonderfully ordered the func- 

 tions of these organs. 



A sufficient, but, at the same time, a definite, quan- 

 tity of light, not too intense for distinct vision, is pro- 

 vided in two modes : — First, according to the greater 



or less intensity of the rays, a greater or less number 



— ■ — 1 



• Nev. Maskelyne, Attempt to explain a Difficulty in the Theory of Vision, 

 depending on the different Refrangibility of Light. Philosophical Transactions. 

 Vol. lxxix. p. 256. 



t J. H. Voiffht, Magasin fir Physik und Naturgeschitchte. T. v. P. iii. 

 p. 143. 



