266 THE SPECIAL SENSES 



necessary that the retinas of the two eyes should corre- 

 spond so exactly that the corresponding fibres in each 

 retina may receive the light from exactly the same 

 point in the object. The nervous impulses will then 

 blend in the brain to form a single sensation, that is, 

 we shall see but one object in spite of the two images 

 on the retinas. 



Double vision. Where the eyes fail to work together 

 and two images are seen, the fault is probably due to 

 an imperfect control of the convergence of the eyes by 

 their muscles. Extreme instances of this are seen in 

 cross-eyes and in diverging eyes. In all such cases, con- 

 siderable nerve strain may result from the attempt to 

 make the images coincide, and the defect should there- 

 fore be remedied by glasses, or, in more extreme cases, 

 by operation upon the eye muscles. 



EXPERIMENTS AND DEMONSTRATIONS 



Dermal Senses 



Materials: Drawing compasses (any form, not too sharp, 

 may be used) ; scale graduated to millimeters; vessels of cold, 

 lukewarm and hot water ; fine horsehair or straight human hair ; 

 pencil; forceps. 

 Experiments (2 students working together) : 



1) Take straight piece of hair in forceps and, by ascertain- 

 ing greatest length which will give rise to sensation of pressure, 

 determine relative sensitiveness of palm; back of hand; fore- 

 head. 



2) Determine least distance that two points of compasses 

 may be separated and still be recognized as two when applied 

 to finger tip ; back of hand ; back of neck ; tongue, etc. 



3, a) Determine sensations caused by slight pressure of pen- 

 cil point on back of hand. 



b) Determine if cold points are constant in their position by 

 testing from day to day. 1 



1 Care should be taken not to exhaust the sense organs by over- 

 , stimulation. 



