OF SIGHf. 177 



where the optic nerve enters, is proved, by the well- 

 known experiment of Mariotte,* to be nearly insensible 

 to light.(D) The principal focus of the rest of the retina, 

 which must be considered as the chief instrument of 

 distinct vision, falls upon an imaginary axis of the 

 globe, corresponding with the axis of the cornea and 

 of the whole eye. This, however, as Kaestner ob- 

 serves in opposition to Boerhaave, is not to be under- 

 stood as if only one point of an object could be seen 

 distinctly at once, while the eye is fixed, and that, to 

 behold another point, the axis of the eye must be 

 changed; for the sensation of a complete object is 

 simple and complete, f 



278. 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 ; £ and 

 of children, who seldom acquire this facility of motion 

 before the third month. 



279. To habit we must ascribe also the circumstance 

 of beholding an object singly, although we have two 

 eyes.§ For infants at first see double, and the double 

 vision which occasionally remains after certain dis- 

 eases of the eyes may be removed by practice and 

 experience. (E) 



280. The combined power of the two eyes does not 



* Troxlcr speaks of this at large, 1. c. T. ii. P. ii. p. 1 , 



f In Optica Qiuedam Boerhaavii et Halleri Commentatur Abr. Gotth. 

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



% See Giov. Bartolozzi, sopra una cieca nata guarita. Verona. 1781. 8vO. 

 p. 99 sq. 



§ W.C.Wells, Kany upon single vision with two eyes. Lond. 1792. 8vo. 



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