chap, xxviii.] ENDOSCOPE. 355 



small concave lenses. Each disc can be fitted on an 

 axis at the back of the ophthalmoscopic mirror, and 

 can be so revolved that any one of the small lenses 

 can be brought directly over the small opening 

 through which the observer looks. If the observer be 

 short-sighted, he can thus bring in front of his eye a 

 small concave lens of sufficient focal length to correct 

 his short sight ; if long-sighted, he puts on the 

 ophthalmoscope the disc of convex lenses, and corrects 

 with one of them. Similarly, if the observed eye be 

 short or long-sighted, the retinal image could not be 

 brought into focus with the mirror only, but the 

 observer can adjust his concave or convex disc, as the 

 case may be, and find a lens that will correct the 

 short sight or long sight of the eye he is observing. 

 In this way the ophthalmoscope may be made a test of 

 the normal degree of refraction of an observed eye, and 

 a measurer of the degree of variation from the normal. 

 Endoscope is the term applied to an instrument, 

 devised in 1853 by Desormeaux, specially for illu- 

 minating the canal of the urethra. It consists of a 

 straight, hollow, metallic sound w r hich is passed into 

 the urethral canal. The outer end is terminated in a 

 wider tube, at the end of which is an eye-piece, 

 through which an eye may look into the sound. Near 

 to the outer end of the tube is a plane mirror, set at an 

 angle, and perforated by a small opening in the centre. 

 Opposite to the mirror a tube comes off at right 

 angles, in which is a plano-convex lens. The rays 

 from a lamp, placed to the outside of the lens, are 

 caught by the lens and concentrated on the mirror, 

 whose angle is so adjusted that the light is reflected 

 into the sound. The canal is thus illuminated, and 

 rendered visible to an eye at the eye-piece, owing to 

 the opening in the centre of the mirror. In the in- 

 strument of Desormeaux, the lamp is in one with the 

 sound and other parts, and the light is placed in the 



