10 REPORT—1846, 
it is to be attached. By this means the cone of rays proceeding from the object-glass 
towards the eye-glass is intercepted by the dark glass at a considerable distance from 
its focus, or most heating point, and thus the heating power of the rays, being spread 
over a large surface of the dark glasses, passes through without injuriously heating 
them, and enters the eye in a cool and agreeable temperature. Another benefit de- 
rived by this arrangement is, that the cell holding the dark glass may be made to con- 
tain several glasses; and those may be of different colours; whereby an opportunity 
is afforded of repeating the valuable experiments of Sir John Herschel on transmitted 
light through different coloured media; and also attempering or adjusting the inten- 
sity of the light entering the eye to the sensibility of the retina. Another benefit is 
obtained, that of using various magnifying powers with the same dark glass arrange- 
ment with the greatest facility. 
On the Meteorological Observations at Kew, with an Account of the Photo- 
graphic Self-registering Apparatus. By F. Ronaups, F.RS. 
Mr. Ronalds, on presenting his third annual volume of observations and experi- 
ments made at the Kew Observatory, described his experiments on the photographic 
self-registration of the electrometer, the barometer, the thermometer and the declina- 
tion magnetometer ; explained his existing apparatus for these purposes, and exhibited 
the resulting photographs, but first briefly adverted to his previous proposals in 1840 
and 1841, and experiments in 1844, relative to the subject. The principal charac- 
teristic of his improved system is a peculiar adaptation of the lucernal microscope. 
An instrument of this kind was employed in July 1845 to register the variations of 
Volta’s atmospheric electrometer. The pair of straws were properly insulated and 
suspended within the body of the microscope and towards its object-end. A con- 
densing lens was placed at the end itself, and a good lamp stood beyond it. A strong 
light was therefore projected upon those sides of the straws which were turned towards 
the condensing lens, and the other sides were in deep shade. The light also impinged 
upon a little screen fitted into the back of a case about two feet long, fixed to the eye- 
end of the microscope at right angles with it, and vertically. Through this screen 
was cut a narrow curved slit whose chord was horizontal and radius equal tothe length 
of the straws, Between the electrometer and the screen a combination of achromatic 
lenses by Ross was adjusted to produce a good chemical focus of the electrometer, at 
a distance as much beyond the external surface of the screen as the thickness of one 
of the plates of glass to be presently mentioned. In the long vertical case was sus- 
pended a frame about half the length of the case, provided with a rabbet, into which 
two pieces of plate glass could be dropped, and these brought into close contact by 
means of six little bolts and nuts. The frame could be removed at pleasure from the 
line by which it was suspended, and the line, after passing through a small hole stopped 
with grease at the top of the long case, was attached to a pulley about four inches in 
diameter on the hour arbor of a clock. Lastly, counterpoises, rollers and springs 
were used for ensuring accurate sliding of the frame, &c. A piece of Mr. Collen’s 
photographic paper was now placed between the two plates of glass in the moveable 
frame ; the long case was closed so as to prevent the possibility of daylight entering it, 
the clock was started, and the time of starting was noted. All that part of the paper 
which was made to pass over the slit in the screen by the motion of the clock, became 
now therefore successively exposed to a strong light, and was consequently brought 
into a state which fitted it to receive a dark colour on being again washed with the 
usual solution, excepting those small portions upon which dark images of the lower 
parts of the straws were projected through the slit. These parts of course retained 
the light colour and formed the long curved lines or bands, whose distances from each 
other, at any given part of the photograph, i. e. at any given time, indicated the 
electro-tension at that time. Sometimes daylight was used instead of the light from 
a lamp, and in that case, during the process, some appearances of the sky were occa- 
sionally noted, by which it was evident that in serene weather, when the sun’s light 
and heat varied, and the paper became consequently either more or less darkened, 
the electric tension, as shown in the photograph, varied also, increasing with the in= 
. crease of light, &c. This fact has not perhaps been before observed, but as the dark- 
ening effect on the paper could not be always depended upon, separate notes were 
taken of the intensities of light and the same results obtained. At the suggestion of 
