If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form.
Begia tresna optiko batekin konparatu behar badugu, ehun gardenezko geruza lodi bat imajinatu behar dugu likidoz beteriko lekuneekin eta behealdean argiarekiko nerbio sentibera bat duela, eta orduan suposatu behar dugu geruza honen parte guztiak etengabeki astiro-astiro dentsitatez aldatzen ari direla, harik eta dentsitate eta lodiera diferenteko eta bata bestetik distantzia ezberdinetara jarririko geruzetan banatu arte, geruza bakoitzaren azalera taigabe ari delarik formaz pixka bat aldatzen.