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 cone ; the zodiacal band ; and the 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 shadoiv 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 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 cei tral 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° 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 :it 1 ight 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 ccr- 

 1 1 n > 1 ona is so apt to be confounded with portions of it, 

 that it is necessary to take it into account. The horizon 

 light is ;i 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°. 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 battd. 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. — Ferric sulphate in solution ,s decomposed 

 by gallic acid, and the resulting black ferri 



* Read before the A. A. A. S., Boston. 



