SCIENTIFIC SUMMARY. 
127 
allowance to be made for the difference between the chemical and visual foci 
of the object-glass. An attempt was made by the exposure of one plate for 
a quarter of an hour, to obtain the darkened limb of the moon, but no im- 
pression was obtained. But the most magnificent photograph of the moon 
yet taken is said, on the authority of some of our scientific contemporaries, 
to be one taken in America, in March last, by Mr. Lewis Rutherford, an 
amateur astronomer of repute. This production was exhibited at the last 
seance of the Academy of Sciences, Paris, by M. Foucalt, and excited great 
interest. The Photographic News has published some curious statements 
with regard to the instrument with which this photograph was taken. These 
inform us that Mr. Rutherford was at considerable pains to spoil a costly 
telescope, by taking out and re-grinding the lenses, in order to render their 
chemical and visual foci perfectly coincident. Considering the simple and 
easy methods by which such an end might have been gained, without making 
any such “ alarming sacrifice,” we think such assertions must have been based 
upon some mistake or misconception. The photograph represents the satel- 
lite with one half the illuminated surface turned towards the earth. The 
surface is extremely rough, and a series of immense cavities correctly indi- 
cated with light and shade, having raised edges, and some lofty cones rising 
abruptly from near the centres of the hollows, form a kind of boundary-lire 
between the illuminated and the shadowed sides, the character of which is 
very clearly marked and peculiarly interesting. 
Registration of Earth-Currents . — The Astronomer Royal and Mr. Glaisher, 
of the Greenwich Observatory, have recently had some improved apparatus 
constructed to register by photography the power and direction of earth- 
currents, which we believe will be shortly tested. Of these electrical effects 
and their causes, very little is at present known, although the electric 
telegraph has fully demonstrated the frequency and power of what are called 
magnetic storms. The present mode of registering these currents is thus 
described : — Paper sensitive to light is fastened round a cylinder of polished 
ebonite, which withstands chemical action. This being placed in a dark box 
horizontally, is made by clockwork to revolve once in the twenty-four hours. 
A ray of gas-light which has passed through naphtha, shines through a hole in 
the lid of the box upon the centre of the slowly moving cylinder. Two wires 
running, the one to Croydon and the other to Dartford, are brought into this 
box, and connected with an astatic galvanometer. The one wire hangs as 
nearly as possible in the magnetic meridian, and the other at right angles to 
it. The earth currents are found to do the same with this apparatus as they 
do with the earth-plates and attached telegraph-wires, thereby moving the 
prepared astatic needles and photographing themselves on the sensitized 
paper. A small mirror is attached to each astatic galvanometer, which 
moving with the needle, reflects a ray of light from side to side on one half 
of the sheet of paper, and thus registers the direction of the magnetic meri- 
dian on one side, and currents at right angles to it on the other, with a fixed 
line between. * The new and improved apparatus is from a design by Mr. 
C. F. Yarley, engineer-in-chief to the Atlantic Telegraph Company. We 
hope to describe it in our next. 
