35 



PHYSIOLOGICAL PHYSICS, [Chap, xxvin. 



the observer, and pulled slightly forwards and down- 

 wards. The rays from the concave mirror are thus 

 brought to a focus in the fauces, the proper position 

 Tbeing secured by adjusting the position of the lamp, 

 and by movements of the head of the examiner, who 

 sees the brightly illuminated fauces, one 

 eye looking through the opening in the 

 mirror. 



The other and essential part of the 

 apparatus (Fig. 158) consists of a small 

 plane mirror, which may be round, oval, 

 or square. Passing off directly from the 

 edge of the mirror is a long stem, which 

 makes an angle with the mirror of about 

 125, and terminates in a handle. The 

 mirror is to be placed in the fauces of the 

 person whose larynx is being examined. 

 Before introduction it is heated to the 

 temperature of the body to prevent the 

 breath of the patient depositing moisture 

 upon it, and so obstructing the view. 



The plane mirror being placed in 

 the patient's fauces, the light from the 

 concave mirror is focussed upon it, and 

 its position is then so adjusted that its reflected 

 rays pass down into the larynx, which may thus 

 be brightly illuminated. This position is usually 

 secured when the plane of the mirror forms an angle 

 of 45 with the horizon. Now the larynx being 

 illuminated just acts as any luminous body, and from 

 its various points rays pass upwards and fall on the 

 plane mirror. From that mirror they are reflected, 

 and, if it be in a proper position, they pass straight 

 outwards to the observer's eye, who thus sees an image 

 of the larynx as if behind the plane mirror, as de- 

 scribed on page 302. Usually the first parts to come 

 into view are the back of the tongue and tip of the 



Fig. 158.- 

 Laryngoscopic 



Mirrors. 



