. The Ontario high school physics. rture and as near to the eyeas possible (Fig. 424). It is clear thatin this case the image* on the retina iserect, and yet it seems to be shows that the brain recognizesas the highest part of an object thatwhich gives rise to the lowest part ofthe image on the retina. 409. Accommodation. The eye when at rrst is adjusted sothat parallel rays entering it are focussed on the retina, that is, it isadjusted for viewing distant objects. Under these circumstanceslight from an object near at hand would be brought to a focusbehind the retina (§§387, 388)
. The Ontario high school physics. rture and as near to the eyeas possible (Fig. 424). It is clear thatin this case the image* on the retina iserect, and yet it seems to be shows that the brain recognizesas the highest part of an object thatwhich gives rise to the lowest part ofthe image on the retina. 409. Accommodation. The eye when at rrst is adjusted sothat parallel rays entering it are focussed on the retina, that is, it isadjusted for viewing distant objects. Under these circumstanceslight from an object near at hand would be brought to a focusbehind the retina (§§387, 388). But when we wish to see an objectclose at hand we involuntarily alter the curvature of the surfaces—chiefly the forward surface—of the crystalline lens, making it moreconvex, so that the image is brought upon the retina and we see itdistinctly. This alteration of the converging power of the eye toadapt itself for near or distant objects is known as accommodation. * It is not a true image, but a shadow cast upon the Fig. How to show that theimage on the retina is inverted. 358 OPTICAL INSTRUMENTS In order to see an object distinctly we naturally bring it near tothe eye. As it approaches, our vision of it improves until it getswithin a certain distance, and then we have to strain tlie eye to seeit clearly, and when it gets too close the image is blurred. Theshortest distance from the e\e at which distinct vision can beoVjtained without straining the eye is known as the least distance ofdistinct vision. This distance for persons of normal vision is from25 to 30 cm. (10 to 12 inches). The magnifying power of an optical instrument depends on thisquantity, and in calculating the magnification it is taken as 25 10 inches, although as a matter of fact it is quite variable withdifferent eyes. 410. Why we have Two Eyes. The images of a solid object, formed on the retinas of the two eyes, are not identical. On accountof the distance between the eyes the right
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