7 o 4 



THE SENSES 



about us we have from long experience and intimate contact learned their 

 actual size and the character of the retinal image formed at definite but known 

 distances. When such an object forms an image of the common size and 

 usual distinctness on the retina, the judgment as to its distance is quickly 

 made. 



In the case of unknown objects which are associated with known ob- 

 jects, the judgment of the size and distance of the latter is used in forming 

 a judgment of the size and distance of the former by comparison. Many 

 visual deceptions are based on these comparisons, a fact that is often taken 

 advantage of by photographers. It is also well known that people living in 

 a moist, hazy climate are utterly unable accurately to estimate distances 

 when suddenly transferred to a clear mountain climate. 



LABORATORY DIRECTIONS FOR EXPERIMENTS ON THE 

 SENSE ORGANS. 



i. Touch. Use the small compasses with rounded tips provided for 

 the purpose, and determine the power of localization of the sense of touch 

 as follows: Have the person observed close his eyes, then touch different 

 parts of the skin, of the hand, arm, face, neck, etc., and let the observed one 

 announce the exact point touched. 



The localization can also be determined by touching two points on the 

 skin with the points of the compasses separated by varying distances. Ex- 

 amine especially the skin on the forearm, on the back of the hand, on the 



FIG. 480. Aristotle's Experiment. 



palm of the hand, the tips of the fingers, and at different points on the face, 

 including the lips and tip of the tongue. Touch these regions of the skin 

 with either one or with two points of the compasses, and allow the person 

 observed to announce results, drawing your conclusions according to the 

 principle of trial and error. Make a table showing the power of local dis- 

 crimination in the different regions. 



