lected by Huygens and ignored by Newton, was destined, in the hands of Young and Fresnel, to afford demonstrative proof of the truth of the hypothesis roughly sketched by Hooke. In his "'Micrographia" (just- ly styled by Pepys " a most excellent piece ") he described, besides a series of beautiful observations with the microscope, the phenomenon known in optical treatises as the " colors of thin plates," and with sin- gular sagacity declared it to form the experimentum cruris as regards chromatic light. These " fantastical " tints (which we may recognize every summer's day in the iridescent glancing of some insect's wing) Hooke diligently examined in soap-bubbles, in " muscovy-glass " (mica), in metallic films, and other similar substances. His explanation of what he observed contains a remarkable, although necessarily imper- fect, approximation to a cardinal truth in optics. By a double reflec- tion from two closely adjacent surfaces, he tells us,* the rays of light are broken up into " confused or duplicated pulses," changing in tint with the varying thickness of the reflecting film. Thus, " colors begin to appear, when the pulses of light are blended so well and so near to- gether that the sense takes them for one." f According to the modern doctrine of " interference," waves of light, pursuing each other at the distance of half an undulation, mutually destroy each other, and pro- duce darkness. But, because difference of color means difference of wave-length, a doubly-reflecting surface, by destroying or reenforcing, according to its varying thickness, undulations of certain lengths, ana- lyzes white light into the prismatic rays of which it is composed, and thus produces the appearances characteristic of " thin plates."
The flaw in Hooke's theory was his erroneous idea as to the nature of color. And on this point we are unable to defend him from the charge of culpable ignorance. The true view was proposed to him } and he deliberately rejected it. The keystone of the arch he had attempted to build was offered to him, and he declined to set it in its place. On February 8, 1672, Newton's memorable paper on the com- position of white light was read before the Royal Society. Had Hooke frankly accepted the discovery, and applied it as a bulwark to his own tottering hypothesis, his name would doubtless have sounded louder in the ears of posterity. But here his moral failings, as well as his intellectual shortcomings, interposed. He was, primarily, an ex- perimentalist. His delight was rather in the things than in the thoughts of Nature. The intimate relations of objects were of less account in his eyes than their external operation on the senses. Add to this the utilitarian tendency impressed upon physical researches by the Baco- nian precepts. In the preface to the " Micrographia " Hooke described as follows the purposes of the Royal Society: " They do not wholly reject experiments of mere light and theory, but they principally aim at such, whose application will improve and facilitate the present way of manual arts." And similar declarations were made by Boyle and
- " Micrographia," p. 66. f "Posthumous Works," p. 190.