9. On the Measurement of the Intensity of Light, by M. Peclet.
In both these methods, the apparent intensity of the shadow varies with the position of the observer. If the shadows are equal when observed from a point perpendicular to the white screen at the middle of the distance of the two shades, they will be no longer so on removing from that position, and the shadow nearest to the observer will always appear the darkest. These apparent variations are greater as the shadows are farther apart, or with reflected shadows as the screen is smoother, or with transmitted shadows as the interposed obstacle is more diaphanous.
The explanation given of this fact is, that unpolished opaque bodies, like paper, plaster, &c. never disperse the light incident upon them, in an uniform manner, more rays passing in the direction in which regular reflexion would take place, than in any other. Hence, when two equal shadows are produced upon such a surface, either by two equal lights at equal distances, or by two unequal lights at unequal distances; the shadow nearest to the observer must necessarily appear deeper than the other, because it is enlightened by the nearest light, the rays from which are reflected in greatest abundance away from the observer; and, on the contrary, the shadow further from the observer should appear lightest, because the rays which fall on it from the furthest light are reflected in greatest abundance towards the side on which the observer stands. The reason, also, why the effect is greater as the shadows are further apart is evident; and why in every case it is reduced to nothing when the observer is in a plane perpendicular to the screen and equidistant from the two shadows.
From these facts and explanations it may be concluded, that, in all photometrical measurements by reflected shadows, the screens should have all smoothness removed from them, and the two [p458] shadows brought as near together as possible, and even made to touch or over-lap; or that, when this cannot be done, the observation should be made from a point equidistant from the two shadows. As to the shadows by transmission, the apparent variations of intensity are so great for small changes in the position of the eye, as to render the method altogether inapplicable.—Bull. Univ. A. viii. 248.
10. On the apparent Decomposition of White Light by a Reflecting Body when in Motion.
11. On the Barometer.
12. Easy Method of reducing Barometrical Observations to a Standard Temperature, by S. Foggo.
| Expansion of mercury, from 32° to 212° F. | |
| De Luc | 1-56th |
| Lavoisier and Laplace | 1-55.22th |
| Halstrom | 1-55th |
| Dulong and Petit | 1-55.5th |
| mean, 1-55.43th. | |
[p459]
For 1° of Fahrenheit’s scale, this is equal to 19977.4, or .00010023: which may be called one ten-thousandth, without the most trifling error in practice. The barometric column may, therefore, be reduced to the standard temperature of 32° F. by the following simple rule, which will make a table unnecessary. Before the first three figures of the observed height place two cyphers, multiply by the temperature of the mercury −32°, and subtract the product from the observed height. Example; barometer 30.597, temperature of mercury 74°.