[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 this plane throat-mirror, be reflected outward in a straight line to the eye, which must be in the same horizontal plane with it. This and all the other facts and principles involved can only be understood by a careful inspection of the accompanying figures, which it is hoped will make the subject plain. The throat-mirror is none other than the mouth-mirror of the dentists, and in use by them before Garcia discovered how it might be employed to throw light on the larynx, in a double sense. The essentials, then, for a view of the interior of the larynx are: A source of illumination; a mirror to reflect the light reaching it from this source into the back of the throat and larynx; and a second mirror to reflect the light outward which is, in the first instance, reflected from below, from the interior of the larynx. The principles involved are few and simple, but their application to any particular case is not easy, and is sometimes well-nigh impossible. The throat-mirror should be placed against that curtain suspended in the back of the mouth cavity known as the soft palate, so that it must be pushed back out of the line of view. But many persons find such a foreign object in the throat a sufficient cause of unpleasant sensations so that retching may be the result. Generally there is a tendency to raise the tongue behind in a way fatal to a view of the mirror and the picture reflected from it. These difficulties, however, can be overcome by a deft hand using the mirror brought to "blood heat" by placing it in warm water or holding it over some source of heat, as a small lamp, and directing the subject observed to breathe freely and _through the mouth_. This latter tends to quiet that unruly member, the tongue, and lead it to assume the flat position so important to an unobstructed view. It is for the same reason the author urges mouth breathing during speaking and singing. No other tends so well to put the tongue in the correct position.