PHOTOMETER (Gr. ydroN, light; metroh measure), an instrument for measuring the intensity a light. The first who occupied himself in scientifically determining the inter). shy ef fight was Bonner; but his investigations were far surpassed by those of Lam bert. about 1760. The latter indicated an exceedingly simple and effective kind of photometer, which was afterwards constructed by Rumford. The instrument consists of a screen of thin paper placed vertically, and behind it, at the distance of a few inches, is placed a cylindrical stick, or any other similar body. When the intensity of light from two flames is to be compared, they are placed behind this stick in such a way that each casts a separate shadow of the stick upon the paper screen. The observer stands in front of the screen, and directs the removal of the two lights either to or from the stick, till the shadows which are cast upon the screen are equally obscure. The distance of each light from the shadow it casts the screen is then measured, and the squares wf these astances give the relative intensities of the two lights. This photometer may also be modified by instead of a cylindrical stick, a second screen parallel to the first, but of greater thickness,'and having an aperture cut in its center. The two lights being then placed behind the second screen, and considerably apart, each casts a sepa rate illumination through the aperture in the second upon the first screen, and the obser ver in front of the latter changes their relative distances till the illuminations appear to the eye of equal intensity. The calculation is the same as before. There are several other classes of photometers, which, however, do not deserve the name, as they depend upon the heating and chemical powers which generally accompany light, and not upon the intensity of its action on the organs of vision. Leslie's instrument is milling more than a differenOal thermometer, while Saussure's and Landriani's depend upon the chemical effects of light. Lampadius, instead of calculating the intensities from the different distances of the lights 1 rem the screen, used plates of born, or other semi-opaque :material, of various thicknesses, and deduced his results from the comparative thickness of the two plates. The results attained by the aid of the photometer, owing to the imperfection of the instrument, are to be relied upon only within certain limits. Some of them are as follows: the light of the sun is 94,500 times greater than that of the moon ; and an ordinary Argand lamp, with cylindrical wick, is equal to 9 newly-trimmed candles.
a new art, invented, during the year 1867, by M. WilThme, Frenchman. It was introduced into Great Britain, practiced by AL Clandet in London. and a society was formed for carrying it tint in Paris; but it was not a commercial sue cess. It consists in taking likenesses in the form of statuettes and medallions by the all of photography, and a very ingenious series of accessory contrivances. A building specially adapted for the purpose is absolutely necessary: this consists of a circular room, 40 ft. in diameter, and surmounted by a glass cupola 22 ft. high, the supporting wall heing about S ft. in height, and pierced with 24 equidistant holes about 4 ft. from the floor; these are only sufficiently large to permit the action of an ordinary camera lens through eath one. Outside the surrounding wall of this circular chamber is it covered dark passage, in which 24 cameras arc placed with their lenses adjusted to the holes in the wall. The person whose likeness is to be taken stands in the center under the glass dome, and at a given signal the cameras are simultaneously brought into action, and a photograph is taken.
The 24 photographs are carefully numbered, so that no error can take place in the subsequent operation, which is performed in another chamber: any room which can darkened will do. It consists in placing them in consecutive order on a vertical wheel. which is so arranged that at the will of the operator each one can be brought before the lens of a magic lantern, and its image projected on a transparent screen. The modeling clay is so pineeil, rather behind the screen, that the artist can use a pantograph, which its reducing point armed with a molding or cutting tool instead of a mere marker; and, as the longer arm of the instrument describes the outline of the projected obtained from tla. photographs, the shorter one is reproducing on a smaller scale 'I he figure in the clay. The statuette thus produced requires retouching with the band to remove the i,luirp and rugged lines of the entting-tools, and of course much depends upon artistic skill in doing this. In the skilled hands which have yet had to do with its operations, the arrangement had so marked a success as to promise to produce in time the most satisfactory results.