PASSAGE OF THE RAYS THROUGH THE EYE. 159 



light you know, in passing from one medium to anoth- 

 er of a different density, are always refracted, or bent 

 out of their direction. If they pass from a denser to a 

 rarer medium, as represented in 

 this figure, where the horizontal 

 black line represents the latter, 

 the space above, the former me- 

 dium, and the oblique line, the 

 ray of light, they are refracted 

 from the perpendicular. But in 

 passing from a rarer to a denser 

 medium, the contrary takes place. 

 These changes happen however, only when the rays 

 fall upon the surface of the medium obliquely ; for when 

 they fall perpendicularly to it, their direction is not chan- 

 ged, whatever may be the difference in the medium. 

 You must bear in mind too, that rays of light proceeding 

 from a luminous object, go on constantly diverging ; that 

 this divergency is increased, by suf- 

 fering the rays to pass through a con- 

 cave lens, and counteracted, or the 

 rays made to converge, by interpos- 

 ing a convex lens. The next two 

 figures will illustrate this principle. 

 Now to understand how the rays of 

 f \ light are modified in their passage 



/ \ through the eye, let us consider a 



single luminous beam falling oblique- 

 ly on the cornea. In entering the 

 cornea, it is refracted towards the 

 perpendicular on account of the con- 

 vexity of this part ; so that when 

 rays fall on every point of the sur- 

 face of the cornea, they will be con- 

 verged, and thus the intensity of the 

 light be increased. 



Emily. And in traversing the 

 aqueous humor, this being denser 



