bluish-grpen, i.e. tlu'^e twii culois are brought t(i tlw saiiio fdoiis; Imt if it is to Ix' used for photo<jraphic purposes, it is "oorrected" for two wave-lenstlis, which incliide lliose radiations pos- scssiiij,' the greatest pliotogiaptiic action. There are two defects which a lens may liave, owing to chromatic aberration (q.v.), in tluit the colored images may be at different distances from the lens and that they may be of ditVerent sizes. The second of these defects is in.signiticant if the lens is thin; and the first may be 'ciirrei'tcd," as just described, by combining two thin li'nses. If the lens is thick, or if the lenses of the lens-system are some ilistance apart, the second of the above mentioned errors becomes serious. It may, how- ever, he corrected.
ACHTERMANN, iic'ter-mAn, Theouore
Wii.iiKi.M I ITD'.i - KSS4). A (Jerman sculptor.
In his sculptures he devoted himself principally
to New Testament subjects. While at Rome, in
1841. he prepared a statue of Christ and an
"Ecce Homo" for the Duke of Aremberg. His
most celebrated luoductions are preserved in the
cathedral at lliinster, and consist of a "Pietil"
and a "Descent from the Cross." Another ad-
mirable work is the marble altar on which are de-
picted three episodes from the life of Christ ( in
relief), prepared in I.S73 for the cathedral at
Prague.
A CHULA, a-sho75'la (Portug.). A dance
similar to the fandango (q.v.).
ACHURCH', Janet. The stage name of
Janet Achurch Sharp, an English actress, the
wife of Mr. Charles Charrington. She was born
in Lancashire and first appeared in London at
the Olympic Theatre in January, 1883. In 1887
she joined Beerbohm Tree's company, and
at the Novelty Theatre, June 7, 188!), created
in English the part of Nora Helmer in A Doll's
House. This was the first presentation of an
Ibsen play to the English public. She has since
toured with a (ompany in India and Australia,
and appeared in the United States with Richard
Mansfield (18n,5), and independently. In June,
1897, at the Olympic Theatre, London, she took
the Shakespearean part of Cleopatra to the
Antony of Louis Calvert.
ACHZIB, flk'zib. (1). A Phoenician city
claimed by Asher (Joshua xix:29), but not
conquered (Judges i:IU); the modern Ez-Zib
on the promontory of Ras-en-Nakurah. Aehzib
is mentioned by Sennacherib. (2) A town in the
Shephelah of .Tudah (Joshua xv: 44). Possibly
the modern 'Ain-el-Kezbeh. near Bet-Nettif.
ACIDASPIS,!ls'r-(lris'])rs (Gk. aKi(, akis,
spine -|- (jaTr/f, (/.<i/)i.s, sliicld). A peculiar genus
of triloliites foiiiul in rocks of Silurian and
Devonian age in nearly all parts of the world.
The individuals arc. as a rule, small, and are
.remarkable becau.sc of the spiny ornamenta-
tion of the dorsal shield or carapace. The loba-
tion of the head shield is rather peculiar and
quite unlike that seen in any other genus of
trilobites, the trilobite divisicm being obscured
by a number of supplementary furrows and by
the strong development of two longitudinal false
furrows between the normal dorsal furrows.
The thorax contains nine or ten segments, and
the tail-shield is of rather small size. In some
species a row of slender spines is developed upon
the sides of the head-shield and a long spine
projects from each posterior angle. Besides
these there are often two long straight or curved
spines directed upward and backward from the
middle posterior edge of the head. Each .seg-
ment of the thorax is produced laterally into
long spines, and there are also two short spines
on the raised median portion of each segment.
The tail-shield is in nearly all species likewise
furnished with spines, so that on the whole
these animals must, though of small size, have
presented a rather formidable aspect to larger
animals which sought to prey upon them. A
few species of the genus are of particular interest
on account of the abnormal development of the
eyes, which are placed at the summits of highly
elevated slender, though immovable, stalks,
which arrangement enabled the animal to com-
mand a view in all directions. This elevation of
the eye recalls the stalk-eyes of some nifidern
crabs and lobsters. For illustration, see Plate of
Tbilobites.
ACIDIMETRY, as'I-dlm'^-trl (Lat. acidus,
sour + tik. /lirpdi; metrnn, measure). The
determination of the amount of acid contained
in a solid or liquid substance. When the cnm-
])ound is a solid, the determination is usually
made by the gravimetric method, which consists
in the dissolving of a known weight of the
material, and its sub.sequent treatment by such
reagents as will yield an insoluble compound,
from the weight of which the amount of
acid can be calculated. When the substance is
a liquid, free Irom foreign matter, the proportion
of acid may be ascertained by determining the
specific gravity of the solution by means of a
hydrometer, but in case of mixtures the acidity
of a solution is best ascertained by the volu-
metric method, which is described under Alkali-
METEU.
ACIDS, fisldz (Lat. acidus, sour). A large
and important class of chemical substances.
They all contain hydrogen, part or all of which
is replaced by metals when the acids are brought
in contact with metallic hydroxides. The com-
pounds formed by substituting metals for the
hydrogen of acids are termed the salts of those
metals, and therefore the acids themselves may
be regarded as salts of hydrogen. An example
inav render these definitions more clearly intel-
ligible. When the sour principle of vinegar is
brought in contact with potassium hydroxide,
a reaction ensues, resulting in the formation of
a new substance. A chemical analysis, com-
bined with a determination of the molecular
weight of the sour principle of vinegar, shows
that the molecule of the latter must be repre-
sented by the formula CJI.O,: on the other
hand, the substance fiu-mcd with ]iotassium
hydroxide is represented by the formula CJl.KO,.
Evidently, part of the hydrogen of the sour prin-
ciple of vinegar has been replaced by the metal
|iotassium ( K ) . We theref(ue class the sour
principle of vinegar with the acids (it is the well-
known acetic acid): and wr class the substance
obtained by its action on potassium hydroxide
with the salts (it is called the acetate of potas-
sium, while acetic acid itself may be called the
acetate of hydrogen).
Most acids have a sour taste and change the blue color of litmus to red. These properties, however, are not strictly characteristic of acids, silicic acid, for instance, possessing neither, thoush — like a true acid — it combines with metallic hydroxides to form salts.
