i8o 
SCIENCE. 
NOTE ON THE ZODIACAL LIGHT * 
By Henry Carwill Lewis. 
The results of a series of observations upon the zodiacal 
light made by the writer, extending over a period of nearly 
five years, is here recorded. The special precautions taken, 
both to train the eye to detect faint lights, and to prevent 
bias on the part of the observer, were given in detail. The 
zodiacal light may be divided into three portions — the 
zodiacal cane ; the zodiacal band ; and th e gegenschein. This 
division is convenient in observation, saves confusion in 
description, and may be in part a natural one. 
The zodiacal cone. — This, the zodiacal light proper, of 
most authors, is the well-known cone of light rising along 
the ecliptic, and best seen in the winter months in the West, 
immediately after the disappearance of twilight. The time 
of shortest twilight coincides with its greatest brilliancy. 
Several observations are given when the writer saw it cast a 
distinct shadow at that time. Its comparative brightness 
with the Via Lactea at different seasons were given, and its 
relation to the ecliptic discussed. It was stated that the 
cone in our latitude is not symmetrical ; and that while its 
axis of greatest brightness lies exactly upon the ecliptic, 
its axis of symmetry is north of that line. An inner short 
cone of greater brightness was described. The warm color 
was shown to be due to atmospheric absorption. No pul- 
sations were ever observed which could not be explained 
either by atmospheric changes or by changes in the eyesight 
of the observer. No periodic changes in the zodiacal light 
were observed ; the same series of changes occurring each 
year with an equal amount of brilliancy. It was shown 
that while the zodiacal cone is frequently seen by moonlight, 
the moon appears to have no appreciable influence upon it. 
The account of the zodiacal cone closes with a description 
of its spectrum , which is always continuous and free from 
bright lines. 
The zodiacal band. — This is an extremely faint zone of 
light, somewhat wider than the Via Lactea, which, like a 
strip of gauze, is stretched across the sky along the zodiac 
from horizon to horizon, and which can be seen at all times. 
It is a belt which forms a very faint prolongation of the 
zodiacal cone, and which, like it, is best seen when the 
ecliptic makes a large angle with the horizon. It is so 
faint that it can only be seen with difficulty. The best 
method of observing it is described. It is brightest along 
an inner line, and fades off more suddenly on its southern 
than on its nothern edge. It has a width of about 12 ", and 
its central line is slightly north of the ecliptic. Observa- 
tions prove the zodiacal band to be a constant and invariable 
phenomenon. 
The gegenschein. — The gegenschein is a faint patch of 
light, some 7 0 in diameter, which nightly appears in that 
part of the zodiacal band, which is 180° from the sun. 
Night after night it shifts its place so as to keep opposite to 
the sun. It is decidedly brighter than the zodiacal band, 
and occasionally a central nucleus about 2° in diameter, of 
greater brightness, can be observed. While the brighter 
portion of the gegenschein is circular, its faint boundaries 
have sometimes the form of an oval, whose major axis is 
parallel to the ecliptic. A large number of maps of its 
position among the stars have been made, which show that 
while its central point is always 180 0 in longitude from the 
sun, it has a latitude of + 2°. 
The moon zodiacal light. — An oblique cone of light in 
the proximity of the moon was described by Rev. G. Jones, 
but has not been detected by the writer. The light preced- 
ing moonrise rises at right angles to the horizon, and seems 
purely atmospheric. One observer has described comet-like 
tails on either side of the moon. The writer holds that such 
appearances are caused by diffraction through floating 
vapor, since they are never seen on clear nights. 
The horizon light. — The phenomenon to which this name 
is applied, though having no connection with the zodiacal 
light, is so continually observed with the latter, and at cer- 
tain seasons is so apt to be confounded with portions of it, 
that it is necessary to take it into account. The horizon 
light is a faint band of light with parallel sides, lying all 
around and parallel to the horizon, and separated from it by 
an interval of darkness. It is brightest, and terminates 
most abruptly on its lower edge. This sharp lower edge is 
5° above the horizon, while the diffuse upper edge varies in 
altitude with the state of the atmosphere. The horizon 
light has a mean width of about 15 0 . It is purely atmos- 
pheric and appears to be caused by reflected starlight. It 
becomes very bright when the moon is above the horizon. 
Below the horizon light is a very dark space here called the 
absorption band. This quenches the light of the Via Lactea, 
the zodiacal cone, and all except the largest stars and 
planets, which last, while in it, are deeply colored. In the 
summer, when the ecliptic is low, the horizon light fre- 
quently blends with the zodiacal band. 
THE ACTION OF SUNLIGHT ON GLASS.* 
By Thomas Gaffield. 
As great a variety of tints and colors appears 'after ex- 
posure to sunlight as is witnessed in the original specimens. 
A general classification of the changes of color produced 
by the sun in colorless glasses is as follows : 1. From white 
to yellowish. 2. From greenish to yellowish-green. 3. From 
brownish-yellow and greenish tints to purple. 4. From 
light-green or greenish-white to bluish. 5. From bluish 
and other tints to darker tints of the same colors. Every 
specimen of colorless glass exposed ten years shows some 
change of color or tint, except some white flint glass, such 
as is used for fine glassware and optical glass. The optical 
glasses with the exception of two specimens of crown, which 
became of a yellowish color, showed only a very slight change 
of tint, leading some to the opinion that oxide of lead, which 
enters largely into its composition, may act as a protecting 
shield against change by sunlight exposure. In experi- 
menting for ten years with colored glasses of the main 
spectral colors (red, orange, yellow, &c.), no change was 
observed in any pot-metal specimens (colored throughout 
the body) save a slight darkening of the purple. A change 
to a purplish or yellowish color was observed in the color- 
less body of some of the flashed and stained specimens, 
when looking through the edges of these glasses, which are 
originally colored on the surface only. The sunlight col- 
oration is not sufficient to be noticed in an observation 
through the surface of the glass. An experiment with pot- 
metals not of the primary colors, but of the intermediate 
ones which most nearly approach those which are produced 
in colorless glass by sunlight exposure, showed the follow- 
ing changes : First, from brownish tints to a flesh color ; 
second, from flesh color to tints of violet or purple ; third,’ 
from amber, olive and purple to darker tints of the same 
colors. 
It is interesting to know that, so far as such colors in 
pot-metal were used in the old cathedral windows, the re- 
sults of these experiments prove that they must have 
changed in color or tint, and that the glass which we see 
in these old churches to-day, and which has suffered sun- 
light exposure for centuries, must be of very different hue 
from that which it exhibited when it left the artist’s studios 
or the glass factories of the mediaeval ages. It is a curious 
fact, noticed by Pelouze and Percy, and confirmed by Mr. 
Gaffield’s experiments, that, with some exceptions among 
the colored specimens, all of the glasses changed in tint 
or color by sunlight exposure can be restored to their 
original color by the heat of a glass-stainer’s kiln, and can 
again be colored after a second exposure to sunlight ; and 
that this coloration by sunlight and de-coloration by heat 
(of about the temperature of red heat) can be carried on 
indefinitely. Diffused light will also color glass, but only 
with a greatly diminished effect, proportioned to its com- 
parison with the power of the direct rays of the sun. 
ON A SOLUTION OF FERRIC GALLATE AND 
FERRIC OXALATE AS A REAGENT FOR THE 
QUANTITATIVE ANALYSIS OF AMMONIA.* 
By Prof. N. B. Webster, of Norfolk, Va. 
Preparation.— F erric sulphate in solution is decomposed 
b) r gallic acid, and the resulting black ferric gallate is par- 
* R before the A. A. A. S., Boston. 
