PRACTICAL EXERCISES 857 



in the sensation may be perceived (sometimes called Weber's law}. 

 Thus, a light of the strength of one standard candle must be increased 

 by i^yth candle, a light of 10 candles by y^ths, and a light of 100 

 candles by a candle, in order that the eye may perceive that an 

 increase has taken place, just as the weight necessary to turn a 

 balance increases with the amount already in the pans. The frac- 

 tion varies for the different senses. It is about T ^ for light, J- for 

 sound. But it would appear that Weber's law does not hold for the 

 pressure sense, nor for the other senses above and below certain 

 limits. Fechner, making various assumptions, has thrown Weber's 



law into the formy=& , where y is the intensity of sensation, 



XQ 



x the intensity of stimulation,^ the smallest intensity of stimulus which 

 can be perceived (liminal intensity), and k, a constant. This so-called 

 psycho-physical law of Fechner states that the sensation varies as the 

 logarithm of the stimulus. But Fechner's law has been subjected to 

 serious criticism, and the subject cannot be further pursued here. 



PRACTICAL EXERCISES ON CHAPTER XIII. 



1. Formation of Inverted Image on the Retina. Fix the eye of 

 an ox or of a dog or rabbit (preferably an albino), after removal of 

 part of the posterior surface of the sclerotic, in a hole cut in a 

 blackened box. Place a candle in front of the eye. Look from 

 behind, and observe the inverted image of the candle formed on the 

 retina. Move the candle until the image is as sharp as possible. 

 Now bring between the candle and the eye a concave lens. The 

 image becomes blurred, the candle must be put farther away to 

 render it distinct, and perhaps no position of the candle can be found 

 which will give a sharp image. If the lens is convex, the candle 

 must be brought nearer, and a sharp image can always be formed by 

 bringing it near enough. If both a convex and a concave glass be 

 placed in front of the eye, they will partially or wholly neutralize 

 each other. 



2. Helmholtz's Phakoscope (Fig. 330). This instrument is em- 

 ployed in studying the changes that take place in the curvature of 

 the lens during accommo- 

 dation. It is to be used 



in a dark room. A candle 

 is placed in front of the 

 two prisms P, P'. The 

 observer looks through the 

 hole B ; the observed eye 

 is placed at a hole opposite 

 the hole A. The candle or 

 the observed eye is moved 

 till the observer sees three 

 pairs of images, one pair, j- IG- 330. PHAKOSCOPE. 



the brightest of all, re- 

 flected from the anterior surface of the cornea ; another, the largest of 

 the three, but dim, reflected from the anterior surface of the lens; and 



