PHYSICAL SCIENCE IX THE MIDDLE AGES. 521 



special virtue of these stones and their hexagonal figure ; lot therefore 

 the experimenter go on, and he will find the same in other transparent 

 stones, in dark ones as well as in light-colored. He will find the same 

 effect also in other forms than the hexagon, if they be furrowed in the 

 surface, as the Irish crystals are. Let him consider too, that he sees 

 the same colors in the drops which are dashed from oars in the sun- 

 shine; and in the spray thrown by a mill-wheel; and in the dew- 

 drops which lie on the grass in a meadow on a summer morning ; 

 and if a man takes water in his mouth and projects it on one side into 

 a sunbeam ; and if in an oil lamp hanging in the air, the rays fall in 

 certain positions upon the surface of the oil ; and in many other ways, 

 are colors produced. We have here a collection of instances, which 

 are almost all examples of the same kind as the phenomena under con- 

 sideration ; and by the help of a principle collected by induction from 

 these facts, the colors of the rainbow were afterwards really explained. 

 ' With reo-ard to the form and other circumstances of the bow he is 



o 



still more precise. He bids us measure the height of the bow and ot 

 the sun, to show that the centre of the bow is exactly opposite to the 

 sun. He explains the circular form of the bow, its being independ- 

 ent of the form of the cloud, its moving when we move, its flying when 

 we follow, by its consisting of the reflections from a vast number of 

 minute drops. He does not, indeed, trace the course of the rays 

 through the drop, or account for the precise magnitude which the bow 

 assumes; but he approaches to the verge of this part of the explana- 

 tion ; and must be considered as having given a most happy example 

 of experimental inquiry into nature, at a time when such examples were 

 exceedingly scanty. In this respect, he was more fortunate than Fran- 

 cis Bacon, as we shall hereafter see. 



" We know but little of the biography of Roger Bacon, but we have 

 everv reason to believe that his influence upon his age was not great. 

 He was suspected of magic, and is said to have been put into close con- 

 finement in consequence of this charge. In his work he speaks of As 

 trology, as a science well worth cultivating. ' But,' says he, 'Theo- 

 logians and Decretists, not being learned in such matters, and seeing 

 that evil as well as good may be done, neglect and abhor such things, 

 and reckon them among Magic Arts.' We have already seen, that at 

 the very time when Bacon was thus raising his voice against the habit 

 of blindly following authority, and seefdug for all science in Aristotle. 

 Thomas Aquinas was employed in fashioning Aristotle's tenets intc 

 that fixed form in which they became the great impediment to the 



