OPTICAL ILLUSIONS. 



327 



distance of only 2 or 3 inches, the curved lines bounding the 

 alternate black and white areas will appear almost straight. 



Fig. 172. Chequers from "Stirling's Outlines of Physiology." 



These and others analogous peculiarities are to the eye very 

 much what the imperfect perception of tempera- 

 ture is to the sense of touch (Expt. 328). A some- 

 what similar example of illusion through the 

 sense of touch is afforded by the effect produced 

 by crossing the first and second fingers, and 

 placing a marble or other hard object between 

 them (fig. 173), when two objects will be appar- 

 ently felt. 



Expt. 358. Double Eefractioa Substances 

 crystallised in certain shapes and of sufficient 

 transparency, when interposed in certain posi- 

 tions between the eye and an object, allow two 

 images to become visible; just as a pane of glass Fig. 173. Aris- 

 interposed obliquely between an object and the to tie's Experiment, 

 eye causes a certain amount of apparent displacement, on account 



