1868. ] Mathematical and Physical Science. 515 
The next paper was “On a new Correction to be applied to 
Observations made with Hadley’s Sextant,” by Mr. T. Dobson, B.A. 
Tt was explained that the sextant is rarely used, save to satisfy the 
practical requirements of nautical astronomy. One object of the 
paper was to show that the instrument might be safely employed 
im determining the angular distance of terrestrial as well as of 
celestial objects, by taking account of a certain correction which 
was here investigated. 
A long paper “ On Actinometry,” by Mr. L. Bing, was then read. 
The instrument used in these researches consists mainly of a tube 
of non-actinic glass, with an arrangement to bring a strip of sensitive 
paper into contact with one side of the interior. This tube is placed 
in diffused light, with its open aperture towards the sky. Light 
entering the tube and falling upon the sensitive paper commences 
at once to blacken it, but its power of so blackening is, of course, 
only in the ratio of the quantity of light which enters, and which 
necessarily diminishes as it proceeds further into the tube. The 
following are the principles upon which this instrument is founded : 
Ist. That diffused light, on entering a tube at one end only, varies 
in intensity within the tube inversely as the square of the distances 
from the aperture where light enters. 2nd. That any number of 
tubes, whatever their magnitude, contain the same intensity of light 
if the ratios of their diameters to their lengths are equal, and if we 
absorb the light that may be reflected from their sides. A carefully- 
calculated scale is fixed inside the tube at the side where the sensitive 
paper is fixed, the divisions being marked by a portion of a faint 
standard tint. When the sensitive paper in the instrument has been 
exposed for (say) ten minutes, the upper portion is darkened, and 
the first two or three tinted marks on the scale look white in con- 
trast with the darkened paper: as the darkening down the tube 
gradually becomes less, the sensitive paper and standard tint appear 
of the same colour, and below that the paper is seen to be white by 
contrast with the standard tint. 
A paper “On the Value of the Hollow Prism in examining 
Absorption Spectra” was then read by Dr. J. H. Gladstone.—In 
the prismatic analysis of the light transmitted by coloured liquids 
or solutions, the author has suggested the use of a wedge-shaped 
trough, so as to produce a line of light that had traversed every 
thickness—from one that was almost opaque to one which was 
next to nothing, and therefore permitted the passage of every ray. 
He now insists on the value of this method, as giving a much fuller 
account of the absorption, and especially as enabling the observer to 
represent it in a characteristic diagram. 
The last paper this day was “On a New Automatic Telegraphic 
Apparatus,” by Professor C. Zenger. 
