276 Description of a Totally Reflecting Prism. 



prevent injury in their passage into the ear. From the middle 

 of the straight handle, and at an angle with it of about 20°, 

 arises a curved branch, e, moveable in a pivot joint at/, toward 

 either side of the handle. This branch forms a stalk, on which 

 a prism of flint glass, g, is perched erect, to the level of the 

 opening of the affixed canula. The prism rests in a metallic 

 socket, and is made to revolve on its own axis at the touch of 

 the finger, or to remain fixed in any desired position by the aid 

 of a small clamping screw, h. The instrument is thus complete 

 for use. The canula is to be introduced into the ear to be 

 examined, the patient being seated by a table, having a good 

 light of a gas jet or argand burner at a convenient distance to 

 one side. The surgeon being placed opposite to the ear to be 

 inspected, a face of the prism is turned towards the light, and 

 it is made to revolve until the luminous spectrum is conveyed 

 to the bottom of the canula, and to the surface sought to be 

 observed. There is no difficulty in the adjustment of position, 

 when the relations of the light and the object are ascertained 

 by a little experience ; and when this adjustment is made, the 

 full and clear illumination of the object is at once obtained, 

 and with a degree of brilliancy proportioned to the quantity of 

 light employed in the particular observation. Where different 

 circumstances require it, the intensity of the light may be arti- 

 ficially increased to any desired extent. 



The principle or theory, as already mentioned, consists in 

 total reflection. The light is received by one side of the 

 prism, is reflected from the second side, and emerges by the 

 third side to the object illuminated, as represented by the 

 dotted lines ac y z^ and thus its view is revealed to the eye. 

 The light afforded is nearly equivalent to the same candle 

 or gas-light applied to the page of a book, or other familiar 

 uses, so little of it is lost in its passage through the prism. 

 The illumination is not preternatural or dazzling, such as 

 would alter the real features of disease, but natural, and 

 such as the eye is familiar with. The advantage of this 

 unconcentrated natural light can only be fully appreciated 

 by professional eyes ; and I am persuaded that any method of 

 concentrating light by lenses or converging mirrors, substitut- 

 ed for the prism, would not increase the serviceableness of the 



