The fundamental ideas upon which photography is based are very old--older than the Christian era, certainly. They depend upon two facts: First--that light, in passing through a small opening, produces an inverted image in a dark chamber. Imagine, for instance, that you are in a dark chamber, outside of which is an object; that there is in the chamber a small hole a sixteenth or an eighth of an inch in diameter, and that you have in this dark chamber a piece of paper. Upon that paper you will get a picture of the object opposite the hole. That was known a long time ago. The other fact is that certain salts of silver, notably the chloride, iodide and bromide of silver, are sensitive to light and become blackened by light, was known to the Egyptians. The action of light upon colored bodies must have been known to the very earliest observers among men. The bronzing of the human skin under the tropical sun must have been noted by every one; and it is on record, in the most ancient annals of the human race, that men--the fair men from the North--when they went to the tropics, returned with tanned skins. Ptolemy, over two thousand years ago, noted that beeswax was bleached in sunlight, and the old Greeks noted that the gems which we call opal and amethyst lost their colors when exposed to sunshine. These are some of the first and most rudimentary notions upon the actions of light, and we have no definite statements about making pictures without light. The Chinese have a tradition--and they have a great many curious ones that are often founded on facts--that the sun makes pictures upon the ice of lakes and rivers.