64 JEROME CAKDAN. 



book upon Light is, on the whole, more than usually ac- 

 curate in its philosophy. Jerome's father had, it may be 

 remembered, studied the subject, and been the first editor 

 of Archbishop Peckham's Perspective 1 . There was a 

 good deal of correct knowledge then afloat concerning 

 optical laws, and by its aid Jerome was ready to correct 

 some popular errors, such as the belief that trees emit 

 sweet odour when the end of a rainbow rests upon them. 

 He knew that rainbows belong to the eye, and have 

 not out of the eye a substantial, separate existence. 



The treatise next passes to substances compounded of 

 the elements, to metallic substances, earths and gems, 

 inquiring, among other things, why amber attracts straws 

 and other light substances, of course without any idea of 

 electricity. He attributes the phenomenon to the fatness 

 and warmth inherent in the constitution of the amber. 

 He then, in his sixth book, treats in detail of the seven 

 metals, and in the seventh book of stones and gems, point- 

 ing out how to tell those that are false, and using some of 

 the knowledge that he formerly obtained from his friend 

 the jeweller, Guerini. He treats also of the properties of 

 gems, and describes three remarkable agates in his own 

 possession. One of them, which he had found to display 

 great virtue in promoting sleep, had incorporated in its 

 substance a profile nearly resembling that of the Emperor 



1 Vol. i. p. 4. 



