A philosophic reader may at once perceive what the Idealist will infer respecting this act of comparison. Neither image--on retina or on screen--exists apart from the eye. So far as we know, if there were no eyes there would be no images; and some writers (_e.g._, Schleiden) have positively affirmed that without eyes all would be, not only to us, but _in itself_, darkness;--the world absolutely void of Light. But the truth may be summed in a sentence. Light is not for the eye in the same sense that the eye is for light. Light is for other things besides. It exerts its activity on life, animal and vegetable;--on inorganic substances;--and in other ways likewise.--Going no further than our screen, we can so manage matters as to engrave and otherwise fix the image thrown upon it;--in other words our moving line of imponderable matter will produce further effects, chemical and mechanical, visible and palpable. Proceeding to a cross-examination of the knowledge with which we have credited ourselves, our next business is to try whether we can verify the objectivity of our optical image. Now it impresses sight in two respects,--as superficial form--and as colour. The family of forms is, we are aware widely connected. Sound evokes them. Draw a violin bow across a string stretched over finely silted sand, and the different notes will be correlated by a diversity of shapes,[96] into which the sand will arrange itself. Therefore, we ought to find means of verifying Form without much difficulty. Indeed we do so every day satisfactorily; our hands are perpetually demonstrating the general accuracy of our eyes, and even those delicate instruments our finger-ends, do not always add much to the information sight has given us.