ant of the actions
which each of the component substances exerts on the propagation of light,
while the absorption of a given region of the spectrum is due to a single
one of these substances, and may have for its directions of symmetry the
directions which it would have in the absorbing molecule supposing it
isolated. It may happen that these directions do not coincide with the axes
of optical elasticity of the compound crystal. If such is the cause of the
anomaly of certain principal directions of absorption, the bands which
present these anomalies must belong to substances different from those
which yield bands having other principal directions of absorption. If so,
we are in possession of a novel method of spectral analysis, which permits
us to distinguish in certain crystals bands belonging to different matters,
isomorphous, but not having the same optical properties. Two bands
appearing in a crystal with common characters, but presenting in another
crystal characters essentially different, must also be ascribed to two
different bodies.

* * * * *

[Continued from SUPPLEMENT, No. 585, page 9345.]

HISTORY OF THE WORLD'S POSTAL SERVICE.

It is commonly believed in Europe that the mail is chiefly forwarded by the
railroads; but this is only partially the case, as the largest portion of
the mails is intrusted now, as formerly, to foot messengers. How long this
will last is of course uncertain, as the present postal service seems
suitable enough for the needs of the people. The first task of the mail is
naturally the collection of letters. Fig. 17 represents a letter box in a
level country.

[Illustration: FIG. 17.--COUNTRY LETTER BOX.]

By way of example, it is not uninteresting to know that the inhabitants of
Hanover in Germany made great opposition to the introduction of letter
boxes, for the moral reason that they could be used to carry on forbidden
correspondence, and that consequently all letters should be delivered
personally to the post master.

After the letters are collected, the sorting for the place of destination
follows, and Fig. 18 represents the sorting room in the Berlin Post Office.
A feverish sort of life is led here day and night, as deficient addresses
must be completed, and the illegible ones deciphered.

It may here be mentioned that the delivery of letters to each floor of
apartment houses is limited chiefly to Austria and Germany. In France and
En