So that when he perceived the odor of the chlorine from the decomposition of the silver chloride, he recognized the gas at once, and he says: “When this silver chloride turns black it gives out chlorine,” and that was a very important fact. At the red end of the spectrum he found there was little or no effect upon the silver chloride. This was the principle of the camera obscura, and the principle of the camera obscura is the principle of the photographic camera to-day. Practically the photographic camera consists of a dark box, with a hole at one end and at this end there is a place to receive an image. Instead of having a lens there in the front of the camera, as was formerly the practice, it is perfectly possible to get the picture with a small opening, say an eighth or sixteenth of an inch in diameter, and, furthermore, that is the most perfect picture you can get in a camera--a picture without a lens. Now, that is a strange statement, and perhaps in these days it may appear a little wild; but (exhibiting a photo about 5 × 7) there is a picture made with an opening not larger than a pinhole, and it is a good deal better than many of the pictures taken by the amateurs to-day. This opening being so small necessitates a good deal of time in the action of the light upon the sensitive silver salts behind, and that is the object of placing the lens there. By placing the lens here, instead of having a small opening, you make a larger opening which collects the light in the same manner, brings it to the focus and then the rays diverge again and you get the picture. Now, the rays as they pass through the opening without a lens, begin to diverge as soon as they are in the camera, but with a lens there they are brought together first and then cross and then you get the picture. That is the first step, then, in photography, the production of images by the camera obscura--and that is all the photographic camera consists of--a modification of it. Now, when the facts ascertained by Scheele, _i.e._, the action of light upon silver chloride--turning it black and producing gas (and by the way Scheele never found out what this gas was and to-day it is a matter of controversy and a problem among chemists)--with the facts ascertained by Scheele, in regard to the action of light Thomas Wedgewood and Vueder made pictures, in 1802. These pictures were very peculiar. They spread upon paper and upon glass plates that had some gummy material upon them silver chloride--as a precipitate, and then they set their subjects up, so as to get a profile shadow with a strong light upon the surface. Now, where the light passed, of course they got a black mark upon the silver chloride, but the silhouette of the face was in white. Now, that was very remarkable, because they got some very remarkable pictures of which drawings were made. They were white silhouettes on a black background, but remember that the pictures that were thus made,