Irradiation of Peacock feathers

Near the end of the sixth book of Browne's Pseudodoxia Epidemica there's a good portrait of  the worthy 17th century Norwich physician engaged upon his essentially Baconian  quest.

At the  conclusion of a chapter entitled A further digression upon blackness there is evidence of the refinement of Browne's senses and  his appreciation of beauty. In  chapter 12 of book  6 Browne alludes to one of his numerous chymical experiments. Several paragraphs are devoted to the use and effects of  various acids and vitriol, the causes of blackness in nature and speculation upon  its causes and origins in human skin. 

The study of optics was also of particular interest to Browne and optical imagery  frequently occurs in  his writings. One strongly suspects his use of the word irradiation, ' to shine brightly' (the near synonymous iridescence is later in origin, 18th century from iris, rainbow) to describe the well-known optical effect when viewing a peacock's feather, is one of Browne's many medical-scientific neologisms. These include the words medical, pathology, hallucination, electricity and ambidextrous, as the complete Oxford English dictionary testifies.

In a tone of near mystical apprehension and barely suppressed joy, Browne concludes his scientific investigations upon blackness, waxing lyrical upon  the beauty of colour thus-.

And this is also apparent in Chymical preparations. So Cinaber becomes red by the acide exhalation of sulphur, which otherwise presents a pure and niveous white. So spirits of Salt upon a blew paper make an orient red. So Tartar or vitriol upon an infusion of violets affords a delightfull crimson. Thus it is wonderful what variety of colours the spirits of Saltpeter, and especially, if they be kept in a glass while they pierce the sides thereof; I say, what Orient greens they will project: from the like spirits in the earth the plants thereof perhaps acquire their verdure. And from such salary irradiations may those wondrous varieties arise, which are observable in Animals, as Mallards heads, and Peacocks feathers, receiving intention or alteration according as they are presented unto the light. Thus Saltpeter, Ammoniack and Mineral Spirits emit delectable and various colours; and common Aqua fortis will in some green and narrow mouthed glasses, about the verges thereof, send forth a deep and Gentianella blew.