Experimental Philosophy. [Lecture 14. 



assimilating the effect of the forces to that of a 

 magnet, which should turn the poles of a series 

 of magnetized needles all in the same direction. 

 When this disposition obtains, the luminous par- 

 ticles are retained in the whole extent of the 

 crystal, and experience no farther motion about 

 their centre of gravity. But there exist other 

 cases where the particles which traverse the 

 crystal are not fixed to a constant position. 

 During all the time of their passage, they oscil- 

 late about their centre of gravity with computa- 

 ble velocities and periods. Sometimes, again, 

 they turn upon themselves, as it were, with a 

 continued motion of rotation. 



The various phenomena, thus briefly alluded 

 to, are classified under the terms fixed and movea- 

 ble polarization. The philosophers above named 

 have established, illustrated, and confirmed them, 

 by a great variety of striking experiments ; and 

 some new instruments (such, for example, as the 

 calorigrade, now sold by opticians) have ori- 

 ginated from these researches. The train of dis- 

 coveries connected with polarization is by no 

 means completed. It has, however, already fur- 

 nished a most striking confirmation of the New- 

 tonian theory of colours, and of the rainbow, 

 establishing their correct accordance with nature 

 and truth, even in the minutest particulars. 



The best account which has yet been given to 

 the world of the discoveries relating to polariza- 

 tion, may be found in the fourth Vol. of Biot's 



