349 



CHAPTER XXVIII. 



OPTICAL INSTRUMENTS. 



THE application of the facts and la\vs relating to 

 mirrors and lenses that have been considered in pre- 

 ceding chapters has resulted in the construction of 

 various instruments of the utmost value in various 

 departments of science. The nature of some of these 

 instruments it is the business of this chapter to con- 

 sider. There will first be described in some detail 

 two instruments of which mirrors form the chief 

 part, and which are of great importance in practical 

 medicine,, the laryngoscope and the ophthalmoscope. 



The laryngoscope is for the purpose of illumi- 

 nating the fauces and pharynx and rendering their 

 inspection more complete, and for making visible the 

 larynx, or, at least, an image of it. The idea of the 

 instrument is due to Listen, the credit of its practical 

 application belongs to Czermak. 



The illumination of the fauces is accomplished in 

 various ways. The usual method is to place the 

 patient opposite to the observer ; at one side of the 

 former, slightly behind him, and on a level with his 

 ear, is a lamp furnishing a steady bright light. 

 Strapped to the forehead of the observer, or supported 

 in a spectacle frame, is a concave mirror. The mirror 

 is pierced in the centre by a small opening, so that 

 when it is brought in front of the observer's eye he 

 can look through the opening. The rays from the 

 lamp are caught on the mirror, and reflected by it 

 into the patient's mouth, which is widely opened, his 

 tongue being held down by a tongue depressor, or by 

 its point being grasped between finger and thumb of 



