Lenses and Waves

194 CHAPTER 5
Things became even more problematic when Hooke turned to the colors
produced by thin films. 100 On the basis of several experiments, he came to
the conclusion that the various colors depended upon the thickness of the
film. 101 In his view, part of the incident light is reflected at the upper surface
of the film, part at the lower surface. Consequently, two pulses following
shortly upon each other are produced of which the second, having traveled a
longer distance, is weaker. The amount of retardation depends upon the
thickness of the layer, thus explaining the variety of colors. In this way,
Hooke had formulated two different, even inconsistent theories of color.
This did not keep him from a generalization:
“That Blue is an impression on the Retina of an oblique and confus’d pulse of light,
whose weakest part precedes, and whose strongest follows. And, that Red is an
impression on the Retina of an oblique and confus’d pulse of light, whose strongest
part precedes, and whose weakest follows.” 102
This explanation ought to encompass both the oblique pulse (with the acute
end being the weakest) and the retarded pulse (with the pulse reflected at the
lower surface of the film being the weakest). It is, however, stated in most
general terms and Hooke did not explicitly consider the question whether his
two mechanisms explaining colors could be made consistent. The
formulation rules out any possibility of mathematization the two individual
theories may have had. Several other inconsistencies and obscurities in
Hooke’s theory can be pointed out. 103
Huygens’ verdict was merciless. The annotations in his copy of
Micrographia make it clear that he did not think much of Hooke. “This must
not be presumed but ought to be demonstrated, …” Huygens wrote in the
margin of the page where Hooke introduced the sine law. 104 He was quick to
point out the sloppiness of Hooke’s reasoning, in particular his use of
‘pulses’ and ‘rays’. 105 In Traité de la Lumière, he mentioned Hooke without
comment, but elsewhere he made no secret of his dissatisfaction. In optics
Hooke had only made ‘shameful blunders’, he wrote to Leibniz in 1694. 106
If Micrographia did not come up to the standards of Traité de la Lumière,
these were not, after all, Hooke’s standards. His goal was not to elaborate a
mathematical, but an experimental theory of colors derived from and
founded on exhaustive empirical evidence. He accepted the sine law as an
empirically founded truth that needed no further mathematical or other
100
Hooke, Micrographia, 65-67.
101
Hooke, Micrographia, 50.
102
Hooke, Micrographia, 64.
103
In his subsequent analyses of refracted pulses, of the refraction of a beam (by a drop of water), his
interpretation of ‘ray’ and ‘pulse’ continuously changed, switching without notice from a microscopic
point of view to a macroscopic and back. Huygens noted several gaps, and some vagueness as well: Barth,
“Huygens at work”, 612-613. See also: Shapiro, “Kinematic optics”, 198-199.
104
Barth, “Huygens at work”, 612.
105
Barth, “Huygens at work”, 612 (in particular 57 II & III).
106
OC10, 612. “… bevues honteuses …”