But Thoreau's little slips of the kind I have called attention to count as nothing against the rich harvest of natural-history notes with which his work abounds. He could describe bird-songs and animal behavior and give these things their right emphasis in the life of the landscape as no other New England writer has done. His account of the battle of the ants in Walden atones an hundred-fold for the lapses I have mentioned. One wonders just what Thoreau means when he says in "Walden," in telling of his visit to "Baker Farm": "Once it chanced that I stood in the very abutment of a rainbow's arch, which filled the lower stratum of the atmosphere, tinging the grass and leaves around, and dazzling me as if I looked through colored crystal." Is it possible, then, to reach the end of the rainbow? Why did he not dig for the pot of gold that is buried there? How he could be aware that he was standing at the foot of one leg of the glowing arch is to me a mystery. When I see a rainbow, it is always immediately in front of me. I am standing exactly between the highest point of the arch and the sun, and the laws of optics ordain that it can be seen in no other way. You can never see a rainbow at an angle. It always faces you squarely. Hence no two persons see exactly the same bow, because no two persons can occupy exactly the same place at the same time. The bow you see is directed to you alone. Move to the right or the left, and it moves as fast as you do. You cannot flank it or reach its end. It is about the most subtle and significant phenomenon that everyday Nature presents to us. Unapproachable as a spirit, like a visitant from another world, yet the creation of the familiar sun and rain! How Thoreau found himself standing in the bow's abutment will always remain a puzzle to me. Observers standing on high mountains with the sun low in the west have seen the bow as a complete circle. This one can understand.