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light, instrument, equal, rays, objects and heat

PHOTOMETER, an instrument intend ed to indicate the different quantities of light, as in a cloudy or bright day, or be tween bodies illuminated in different de grees. The ratio of the intensities of two luminous objects has been attempted to be measured, by placing them at different dis tances from a given object, until that object cast two shadows of equal darkness ; or by observing when twit equal objects appear ed to be equally illuminated each by one of the luminous objects ; tor then, by a well known and established law, t he proportion of the intensities of their light was suppo sed to be as the squares of the distances. Thus if two equal objects appear to be equally illuminated, when one of them is three feet from a tallow candle, and when the other is nine feet from a wax candle, then it is inferred that the intensity of the light of the former candle is to that of the latter as nine to eighty-one. Mr. Leslie has more recently invented an instrument of this kind, the essential part of which con sists of a glass tube like a reversed sy phon, whose two branches should be equal in height, and terminated by balls of equal diameter ; one of the balls is of black enamel, and the other of common glass, into which is put some liquid.

The motions of the liquor, which is sul phuric acid tinged red with carmine, are measured by means of a graduation, the zero is situated towards the top of the branch that is terminated by the enamel led ball. The use of this instrument is founded upon the principle, that when the light is absorbed by a body, it pro duces a heat proportional to the quantity of absorption. When the instrument is exposed to the solar rays, those rays that are absorbed by the dark colour, heat the interior air, which causes the liquor to descend at first with rapidity in the cor responding branch. But as a part of the

beat, which had introduced itself by means of the absorption, is dissipated by the ra diation, and as the difference between the quantity of heat lost and that of the heat acquired goes on diminishing, there will be a point where, these two quantities hav ing become equal, the instrument will be stationary, and the intensity of the incident light is then estimated by the number of degrees which the liquor has run over. The author of this ingenious instrument, has pointed out its advantages in deter mining the progressive augmentation un dergone by the intensity of the light, and the gradation in a contrary sense, which succeeds to that progress, both from the beginning of day to its end, and from the winter solstice to the end of the succeed ing autumn. With the help of such an instrument, one might also compare the action of rays of light in different coun. tries, of which some dart with sufficient constancy from a fine and serene sky, while others seem to be covered with a veil which dims and obscures their lustre. Mr. Leslie, having proposed to himself to measure the energy of the several colour ed rays which compose the solar spec trum, caused a beam of light to pass through a prism of flint glass; and the indications of the photometer presented successively to the different parts of the spectrum, have furnished nearly fiir the relation, between the degrees of force of the blue, green, yellow, and red rays, that of the numbers 1, 4, 9, 16; a relation which, considered in the two extreme terms, is more than quadruple that which was substituted for it by Dr. Herschel, who has made experiments for the same purpose.