This name has been given to an apparatus employed by Bouguer for measuring the intensity of the light which proceeds from different bodies.

In the case of two lamps or candles burning near to each other, the intensity of the light may be compared, by comparing the intensity of the shadows of an adjacent body. The shadow produced by interposing the body in the stronger light is darker than its shadow formed by the interposition of the body in the rays of the weaker; because the first shadow is enlightened only by the light it receives from the weaker light, whilst the other shadow is more strongly enlightened by the light from the stronger light. But this method will not serve for comparing the intensity of the light of the sun and moon, for the light of the sun so greatly exceeds that of the moon, that when both are visible at the same time, the light of the moon is far too weak for any shadow to be formed by intercepting it.

For the purpose of measuring and comparing the intensity of the light of the sun and moon, Bouguer reduced the light of the sun and of the moon till the light appeared to be equal to that of a candle. The eye is able to judge whether or not two lights are equal; but if the lights are unequal, it is impossible to estimate by inspection how much the one exceeds the other. Being able to calculate how much he had reduced the light of the sun and of the moon, he was enabled to say what proportion their light bore to that of the candle.

For the former, Bouguer, when the sun was at an elevation of thirty-one degrees above the horizon, received the light into a dark room, through a hole of one twelfth of an inch in diameter. In this hole was placed a concave glass to weaken the light, by making the rays diverge more than they would in passing through the hole without a concave glass. He received the image formed by the divergent rays at a distance of five or six feet, by interposing a screen, which formed a right section of the cone of divergent rays. This right section was a round image of the sun, of \( \frac{1}{12} \) of an inch in diameter; therefore the light was diminished by the square of that number, since the light received through a hole of one line in Lucimeter diameter was diffused over a circle of 108 lines in diameter, and of 1164 times the area. This light he found to be equal to that of a candle placed at a distance of sixteen feet.

He employed the same concave glass to receive the light of the full moon, the moon being nearly at its mean distance from the earth, and at thirty-one degrees of altitude, the same altitude as the sun had in his observations of that luminary. The light of the moon being weak, he received the image at eight twelfths of an inch from the concave glass, and then the light of the image was so weak as to be equal to the light of a candle at the distance of fifty feet.

Now the light of the moon was diminished in the proportion of the square of one to sixty-four, which is the square of the diameter of its circular image. If the light of the moon had been diminished 11,664 times, it would have been equal to the light of the candle removed to a distance of 675 feet; for as eight, the diameter of the image, is to fifty, the distance, so is 108, the diameter of the image when the light is reduced 11,664 times, to 675 feet, the distance of the candle which would produce that degree of light. Therefore the light of the sun being equal to that of a candle placed at the distance of sixteen inches, and the light of the moon being equal to the light of the same candle placed at the distance of 675 feet, that is, 8100 inches, it follows that the light of the sun is to the light of the moon as 65,610,000, the square of 8100, is to 256, the square of 16. This experiment gave the light of the sun 256,289 times greater than the light of the full moon. Bouguer obtained results somewhat different from other experiments, and taking a mean of these results, he concludes that the light of the sun is 300,000 times greater than the light of the full moon. The light of the moon, when collected into a focus by a concave mirror which condenses the light into a space 306 times less than the natural state of the light, produces no sensible heat.

Bouguer found, by a process similar to that above described, that the light of the full moon when elevated 66° 11', was to the light of the full moon elevated 19° 16' above the horizon, as 2500, the square of 50, to 1681, the square of 41. The sun at the harbour of Croisac in Bretagne, where Bouguer made his experiments, has the same apparent altitudes of 66° 11', and 19° 16' at the summer and winter solstices; and he concludes that the intensity of the light of the sun at the summer solstice is to the intensity of its light at the winter solstice, in the above-mentioned ratio of 2500 to 1651, or about three to two. He found that the light of the moon, when near the horizon, and about to set, was 2000 times less than the light of the moon elevated 66° 11'.

This difference in the light of heavenly bodies when at different altitudes proceeds from the want of transparency in the air; for when the heavenly body is not much elevated above the horizon, the rays proceeding from it have a greater distance of atmosphere to pass through than the rays from the same body at a greater altitude; and the rays from a heavenly body in the zenith proceed in the direction of a diameter, and therefore traverse the atmosphere by the shortest path. Bouguer calculates that the mass of air passed through by the rays from a heavenly body at 66° 11' is equal to a supposed mass of air of the uniform density of the air at the surface of the earth, 4275 toises in thickness; and that the mass of air passed through by the rays from a heavenly body 19° 16' in altitude, is equal to a supposed mass of air of the density of the air at the surface, and of 11,744 toises in thickness.

He concludes that light is diminished \(\frac{3}{16}\), or about one third, by passing through a mass of air of the uniform density of the air at the surface, and 7469 toises in thickness. The rays from a heavenly body near the horizon are likewise impeded by the terrestrial vapours, which vary in density, and render the intensity of the light of a heavenly body at the horizon various at different times. The same cause renders the quantity of the refraction of the heavenly bodies at the horizon uncertain.

In certain states of the air, the discs of the sun and the moon, when at the horizon, appear somewhat elliptic, the apparent vertical diameter being less than the horizontal. This is occasioned by the greater refraction of the air through which the under limb is seen; which refraction elevates the apparent place of the under limb by a quantity sensibly greater than that quantity of refraction by which the upper limb is raised. This alteration of figure, and the colour of the rising and setting sun and moon, indicate the greater or less quantity of vapour diffused in the air, and are usefully referred to by the seaman and farmer as prognostic indications of the weather.

The existence of a greater or less quantity of vapour mixed with the air is also indicated by the different degrees of intensity of the blue colour of the sky. Some are of opinion that this colour depends on the colour of the mass of air which forms the atmosphere; others maintain that it arises from the darkness of space seen through the interposed atmosphere. The blue colour of the sky increases in intensity from the horizon to the zenith, and is particularly intense when seen from the elevated parts of the Alps, because in that situation there are few terrestrial vapours mixed with the air, and it is the white colour of these vapours which renders the blue less intense in lower situations. For the purpose of estimating and noting its intensity, it is compared with different tints of blue painted on a card, as described under the article CYANO-

METER.

See Essai d'Optique sur la Gradation de la Lumière, par Bouguer; et Voyage de Humboldt, Relation historique, chap. iii. p. 251.