Chap, xxviii.] THE OPHTHALMOSCOPE. 351 



epiglottis, and then, as the plane mirror is adjusted, 

 the cartilages of the larynx and the vocal cords. The 

 image is reversed to this extent, that what appears 

 posterior in the mirror is anterior in the patient, and 

 vice versa. But the right side of the image is also the 

 right side of the patient, only, because of the relative 

 positions of patient and observer, the right hand of 

 the observer is opposite the left of the patient, and 

 consequently the vocal cord seen in the mirror to the 

 observer's right is the patient's left vocal cord, and 

 the patient's right cord is to the observer's left. 



An ingenious arrangement devised by the late 

 Dr. Foulis, of Glasgow, permits a person to examine 

 his own larynx. A glass globe, such as is used by 

 jewellers to focus the light on their work, is filled with 

 water and mounted on a candlestick. Above it is 

 placed vertically a piece of plane looking-glass. The 

 person sits down with this on a table in front of him. 

 On the other side of the globe is a lamp. The globe 

 concentrates the light on the person's face. He opens 

 his mouth and allows the light to be focussed on the 

 faucv Q ,s, which he sees illuminated by looking in the 

 mirror. Guided by the image in the mirror, he intro- 

 duces the small laryngoscopic mirror in the usual way, 

 and thus sees in the mirror in front of him an image 

 of the image in the laryngoscope. 



The ophthalmoscope is a small concave mirror 

 by means of which rays of light are directed through 

 the pupil of the eye so that the deep parts are illumi- 

 nated and rendered visible. It was invented by 

 Helmholtz in 1851. The deep structures of the eye 

 cannot usually be seen, because rays reflected from 

 them diverge as from any luminous point at a finite 

 distance. The divergent rays, as they pass through 

 the media of the eye, are converged, and meet in a 

 conjugate focus outside of the eye. The observer, 

 to perceive the image thus formed, must have his eye 



