7 88 THE POPULAR SCIENCE MONTHLY. 



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. 



