NATURAL PHILOSOPHY. 



239 



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the lens placed in the mirror's centre. The retina and the lens form a mi- 

 croscope, the multiplying power of which is about five hundred. 



SPHYGMOSCOPE. 



The above name has been given to an instrument invented by Dr. Scott 

 Alison, of London, for indicating the movements of the heart and blood- 

 vessels. It consists of a small chamber containing alcohol, or other liquid, 



provided with a thin India 

 rubber wall, where it is to be 

 applied to the chest. At the 

 opposite extremity the cham- 

 ber communicates with a glass 

 tube, which rises to some 



Fig. 2 height above its level the 



chamber. Liquid is supplied 

 to the instrument until it 

 stands in the tube a little 

 above the level of the cham- 

 ber: The pressure of the 

 column of liquid in the tube 

 acts upon the elastic or yield- 

 ing wall of India rubber, and 

 causes it to protrude. This 

 protruding part, or chest- 

 piece, is very readily affected 

 by external impulse ; it yields 

 to the slightest touch, and, 

 being pushed inwards, causes 

 a displacement of this liquid 

 in the non-elastic chamber, 

 and forces a portion of the 

 liquid up the tube. The pro- 

 truding wall of India rubber 

 is driven inwards when it is 

 brought in contact with that 

 portion of the chest which is 

 struck by the apex of the 

 heart, and a rise in the tube takes place. When the heart retires, the India 

 rubber wall, affected by the pressure of the column of liquid in the tube, is 

 pressed back, follows the chest, and permits the liquid to descend. The 

 degree to which the India rubber wall is forced in by the tube, and the 

 amount of protrusion of the India rubber wall which takes place when the 

 heart retires is denoted by a corresponding fall in the tube. The tube is 

 supplied with a graduated scale, to denote the rise and fall with exactitude. 

 The glass tube is provided at the top with a brass screw and collar, to 

 prevent the egress of the liquid when the instrument is not in use, or a bulb 



