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Daguerreotypists generally mark time by their watches, arriving at the nearest possible period for producing a good picture by making several trials. As a ready method of marking short intervals of time is, however, a very important consideration, and as any instrument which will enable an artist to arrive at the exact period, must be an improvement, and worthy of universal adoption, I will here describe one invented by Mr. Constable of England, which he calls a Sand Clock, or Time Keeper.--"It consists of a glass tube, about twelve inches long, by one in diameter, half filled with fine sand, similar to that used for the ordinary minute glasses, and, like them, it has a diaphram, with a small hole in the centre through which the sand runs. The tube is attached to a board which revolves on a centre pin; on the side is a graduated scale, divided into half seconds; the tube is also provided with a moveable index. This instrument is attached, in a conspicuous place, to the wall. The glass tube being revolved on its centre, the index is set to the number of half seconds required, and the sand running down, the required time is marked without the possibility of error. In practice it will be found to be a far more convenient instrument for the purpose than either a clock or a seconds watch, and is applicable both for the camera and mercury box." If the artist finds it desirable or necessary to take the object to be copied in its right position, that is reverse the image on the spectrum, he can do so by attaching a mirror (which may be had of Mr. Anthony, or Mr. Roach) to the camera tube, at an angle of forty-five degrees. If, after taking the plate from the camera, it be examined,