134 VISION. 



gated must be entirely at rest, and supported on a horizontal surface. The 

 weights used must be moderate from a pound to four or five pounds j in 

 which case it will be found that a difference between two weights of one- 

 thirtieth can be detected. 



For testing the sensation of locality in any part of the surface of the 

 body, a pair of compasses is used, of which the points are provided with cork 

 sheaths, having smooth blunt ends. The points being at first at such a dis- 

 tance that when both touch the skin or mucous membrane of the tongue, 

 they are distinctly felt as two, they are gradually brought nearer until the two 

 impressions blend into one. The smaller the distance at which this happens, 

 the finer is the sensation of locality in the region investigated. Another 

 method is that of interrogation. The observer touches the skin, and asks the 

 observed person to designate the locality touched. 



The sensation of muscular Exertion is tested by experiments, 

 each of which consists in lifting in succession two weights, of which one is 

 heavier than the other by a small but perceptible difference ; this difference is 

 diminished at each trial until it can no longer be appreciated. As it is essential 

 that sensation of pressure should be excluded, the weight to be estimated 

 must in each trial be enclosed in a handkerchief, of which the corners must 

 be held in the hand. 



For the investigation of the sensation of taste and of the limits of the 

 gustatory region, four test liquids should be prepared, viz., saturated solution 

 of sulphate of quinine, 10 per cent, solution of common salt, 3 per cent, 

 solution of sugar, and o'l per cent, solution of citric acid. These liquids 

 represent the four fundamental sensations, each of which may be tested 

 separately, or two alternately. In each experiment a camel hair pencil is 

 dipped in the liquid, drained by touching it with filter paper, and applied for 

 a moment to the surface. To secure freedom from bias on the part of the 

 observed person, trials should be made in which tasteless liquids, or liquids 

 of different tastes are alternated in various orders, care being taken to irrigate 

 the surface between each trial and the following one, with water. 



The voltaic sensations of taste are experienced when two zinc plates, which 

 form the terminals of a Grove's element, are applied respectively to the upper 

 and under surface of the tongue as far back as possible. As the effect differs 

 according to the direction of the current, a reversing key must be introduced 

 into the circuit. 



VI. Vision. * 



I. The application of Schemer's experiment to the limitation of 

 the range of accommodation can be best understood if it is made as follows : 

 Stretch a white thread from end to end along the blackened surface of a narrow 

 black board about a yard long. Fix at one end of the board a vertical screen 



* The experiments and observations described under this heading are 

 arranged in the order in which the subjects they are intended to illustrate 

 happen to be referred to in the lectures. 



