Page:Philosophical Transactions - Volume 013.djvu/186

 left is scarce so at all, whereby there will be three parts to one more in the distance of the pupil of the right Eye from the external Canthus (as may be judg'd by the proportion of the White that appears) then there will be in the other; so that the position of the right Eye in respect of the left is as in Fig. 5.

Hereby it appears that if the Object be so plac't that it is seen with both Eyes, the right Eye accommodates it self to the position of the left, that the rays strike correspondent Fibres, and the percussion or Vibration being toward the bottom or Papilla of the Eye (or near its Axis) where I before observ'd Vision to be chiefly performed, a small turning of one Eye to another will make that accommodation.

Moreover as this accommodation is made in an oblique position of the object, so is it more readily done in a direct position of the same; and this we may perceive in an Object's retiring in a straight line from the Eyes, whereby the Pupils gradually devaricate; as on the contrary, they converge when the Object is seen very near them, and that so forcibly that 'tis a pain to hold them long in that posture. Now by this various incidence of the rays sometimes on the internal and sometimes external Fibres (according as the Object approaches or recedes from us) its varying position in respect of distance from us, is perceived, tho it recedes from us in a straight line, and at the same time be equi-distant