rvations, it remains
to this day the foundation of our knowledge of the action of the
larynx in voice-production.

[Illustration: FIG. 38 (Bosworth). Intended to illustrate the optical
principles involved and the practical method of carrying out
laryngoscopic examination. The dotted lines show the paths of the
light-rays.]

As usually employed, the laryngoscope consists of two mirrors, the
head-mirror, so called because it is usually attached to the forehead
by an elastic band, and the throat-mirror, which is placed in the back
part of the mouth cavity. The purpose of the head-mirror is to reflect
the light that reaches it from a lamp or other source of illumination
into the mouth cavity so perfectly that not only the back of the
mouth, etc., but the larynx itself may be well lighted up; but
inasmuch as this illumination may be accomplished, under favorable
circumstances, by direct sunlight, the head-mirror is, though mostly
indispensable, not an absolutely essential part of the laryngoscope.
There is, indeed, one advantage in the use of direct sunlight, in that
the color of the parts seen remains more nearly normal. Lamplight
tends, because of its yellow color, to make parts seem rather of a
deeper red than they actually are; but this to the practised observer,
always using the same source of illumination, is not a serious
matter--his standards of comparison remain the same. Moreover, this
objection does not apply equally to electric light, now so much used.

[Illustration: FIG. 39. This illustration is meant to show more
especially the relative position of observer and observed. The
observer, on the right, is wearing the head-mirror, while two
throat-mirrors seem to be in position--in reality, the same mirror in
two different positions. One is placed so as to reflect the picture of
the nasal chambers, especially their hinder portion. The walls of the
nose, etc., may for the purposes of this illustration be considered
transparent, so that the scroll (turbinated) bones, etc., come into
view. The tongue is protruded. The light, not seen in this figure, is
usually placed on the left of the subject, as in Fig. 38.]

It being a fundamental law of light that the angle of reflection and
the angle of incidence correspond--are, in fact, the same--it was
necessary that the throat-mirror should be set at an angle to its
stem, so that the light passing up by reflection from the larynx
should, when striking on the surface of t