The mechanical structure of the eye is very curious, but I shall not describe it now. It is sufficient to say that light is the great instrument by which vision is performed. This is supposed to consist of innumerable particles, inconceivably small, which proceed in straight lines from every part of luminous or shining bodies. These fly with a velocity ten million times as swift as a cannon ball, for they come from the sun to the earth in eight minutes! These rays of light enter the ball of the eye at the pupil; and at the bottom of a cavity in the ball, called the retina, a little picture is painted of every object placed before the eye. It is this little picture that enables us to see; and we see distinctly, or otherwise, as this is clear or obscure. A very curious thing is, that this picture paints everything reversed, that is, upside down. The reason why we do not, therefore, see things upside down, is a matter that has puzzled greater philosophers than Bob Merry. Merry’s Adventures. CHAPTER XXII. The book shop in which I was now a clerk, was not like the present Broadway establishments of Appleton, or Wiley & Putnam--a vast hall, with almost endless successions of shelves, and these loaded with the rich and varied volumes of the American and English press. No indeed! it was a little shop in Pearl street, stocked with Webster’s Spelling Books, Watts’s Psalms and Hymns, Young’s Night Thoughts, Webster’s Third Part, the American Preceptor, and other works of a popular kind, and designed for general use. There were no Rollo works--there was no Peter Parley then! Mr. Cooke was a very sharp man in trade. His whole soul was bent on making money. He cared nothing for books, except for the profit he made upon them. For a few days he left me to myself, but then he began to try to make me as much interested in the business as he was. But this was a vain attempt. My thoughts were always somewhere else, and often when he spoke to me I did not hear him. I was constantly making blunders. In casting accounts I got everything wrong; I credited Mr. Lightfoot with books that should have been charged; I sent off to a customer a lot of Peregrine Pickle, instead of Young’s Night Thoughts; and at last, taking the inkstand for the sand-box, I dashed a puddle of ink over the ledger!