SIGHT. 567 



distance from it. The rays which diverge from d and e will, 

 after undergoing the various refractions, converge to the points 

 d' e upon the retina, and will there form an inverted image of it, 

 precisely in the same manner as a convex lens forms an image 

 upon a sheet of white paper placed at its focus. 



That an image or picture is actually thus formed on the retina, 

 may be shown by scraping away the sclerotic coat of the eye of 

 an ox, until it is sufficiently transparent for the image to be seen 

 through it. 



" Although the whole of the retina is sensible, it is not through- 

 out equally calculated to receive the images of objects. 



" In the first place, the true axis of the human z eye, where 

 the optic nerve enters, is proved, by the well-known experiment 

 of Mariotte a , to be nearly insensible to light. 



" The principal focus of the rest of the retina, and which must 

 be considered as the chief instrument of distinct vision, falls upon 

 an imaginary axis of the globe, corresponding with the" centre 

 of the cornea and of the whole eye. This, however, as Kaestner 

 observes in opposition to Boerhaave, is not to be understood as 

 if only one point of an object could be seen distinctly at once, 

 the eye being fixed, and that, to behold another point, the axis 

 of the eye must be changed; for the sensation of an entire object 

 is simple and complete. b 



" The habit of directing the axes of the eyes rapidly towards 

 objects is acquired by practice. This is proved by the example 

 of persons who were born blind but recovered their sight after 

 puberty c ; and of children, who seldom acquire this facility of 

 motion before the third month. 



" An object can never be seen unless the angle of vision ex- 

 ceeds 34- seconds. This was proved by the very beautiful expe- 

 riments of the acute Tob. Mayer, who formerly was one of our 

 number : and he demonstrated the great perfection of the human 



z " I say the human eye ; for in some animals now before me, the seal and 

 porcupine, for instance, the true and imaginary axes are the same, the optic nerve 

 lying exactly opposite the centre of the cornea and pupil." 



a " Troxler speaks of this at large, 1. c. t. ii. P. ii. p. 1." 



b " In Optica Queedam Boerhaavii et Halleri Commentatur Abr. Gotth. 

 Kaestner. Lips. 1785. 8vo. p. 7." 



c " See Giov. Bortolazsi, Sopra una cieca nata guarita, Verona, 1781. 8vo. 

 p. 99. sq." t 



