1864. 
mirror of 8 inches’ aperture and 
113 inches’ focal length is used. At 
first, when it was intended to em- 
ploy diffused daylight and the whole 
aperture, the figure was made ellip- 
tical, with a distance of 8 feet be- 
tween the conjugate foci ; but sub- 
sequently, when the advantages of 
sunlight were understood, the sur- 
face was reduced by a diaphragm to 
13 inches in diameter, and a part of 
the mirror as nearly perfect as a 
mirror can be made at present was 
selected. Success in enlargement 
becomes with this contrivance a cer- 
tainty. 
The ‘‘enlarger” is also equally 
valuable in copying by contact. 
When a small negative is enlarged 
and photographed, what is termed a 
positive results. If such a positive 
is used to make prints on paper, the 
lights and shades are inverted, and 
that which is white is shown black. 
It is necessary then to turn the 
original negative into a positive, so 
that when magnified a negative may 
result suitable for printing positive 
proofs on paper. This is done 
usually by a process called reversing, 
in which a sensitive plate is placed 
behind the original negative, and 
the two exposed to the light. 
Wherever the negative is transpa- 
rent the plate behind is stained by 
the light, and where opaque, it is 
protected. But unless the plate be- 
hind is so close as to make the 
chances of scratching the negative 
very great, the positive produced 
is much inferior in distinctness, be- 
cause the diffused light of day finds 
its way through in many directions. 
If, however, the negative and sen- 
sitive plate are placed in the beam 
of sunlizght coming from the en- 
larger, the rays pass through only 
in one direction, and the reverse or 
positive is as sharp as the original 
negative. 
Celestial photography is as yet 
only in its infancy. The results to 
which it has given origin, although 
excellent in many respects, have im- 
perfections. But it seems probable 
that these may be overcome in the 
future, partly by means now within 
Notes and Correspondence. 
3085 
reach, and partly by others which 
may be discovered at any moment, 
In looking at a 3-foot photograph 
from such a distance that the eye can 
embrace it all at one glance, the 
general effect is certainly very fine, 
and superior to observation through 
the telescope with a similar power. 
The moon appears as it would if 
viewed from a stand-point 600 miles 
from its surface. Ranges of moun- 
tains, as the Apennines, seem as if 
projected out from the general level, 
while the great craters, such as 
Plato, Theophilus, and Clavius, are 
deeply excavated below. Grooves 
of vast extent, like those diverging 
from Tycho, and faults such as that 
running past Aunt and Catharina 
on the one side, and Tucitus on the 
other towards Lindenau, still further 
break up the surface. The well- 
known seas and bright portions, so 
distinct to the naked eye, are lost 
in the multiplicity of the details 
into which they are resolved. 
But coming more closely to the 
picture, and examining with a cri- 
tical eye, it is apparent that, al- 
though the general effect is the same 
as would be perceived by looking 
at the moon itself, yet some of the 
minute details seen in the telescope 
with a high power are absent. 
The reasons which lie at the bot- 
tom of this difficulty are connected 
to a certain degree with the photo- 
graphic processes employed, but also 
to not a little extent with the con- 
dition of the air. The quality of 
the instrumental means used is, of 
course, of primary importance. A 
good photograph cannot be taken 
with an inferior telescope and clock. 
The obstacles arising from photo- 
graphy result from the fact that the 
dark parts of the picture are not 
formed by a continuous sheet of 
material, but by an aggregation of 
granules which, though invisible to 
the unassisted eye, are seen when a 
high-enough magnifying power is 
employed. Their degree of visi- 
bility turns on the system of deve- 
lopment used for bringing out the 
latent image on the sensitive plate. 
A picture injudiciously forced with 
