Lenses and Waves

1677-1679 –WAVES OF LIGHT 193
Hooke’s causal account of colors is somewhat the odd man out of
Micrographia. The book is a sustained effort to show that the world is made
up of seemingly imperceptible particles and structures and that the newly
invented microscope has enabled manhood to open up these uncharted
world plus ultra the visible surface of things.
Hooke’s theory of waves is found in ‘Observation XI’ of Micrographia
(1665). This section contains an experimental investigation of the colors
produced in thin films of transparent material, like the lamina of ‘Muscovy
glass’, or the space between two lenses pressed together, or soap-bubbles.
According to Hooke, these colors meant a refutation of the explanation of
prismatic colors Descartes had given in Les Météores (1637). The latter had
argued that no colors are produced when there is no net refraction. Hooke
argued that in thin films there is no net refraction, so according to Descartes’
theory no colors should be produced. Yet, observation shows they are. 94 In
addition he argued that according to Descartes’ own theory no colors would
be produced in rain-drops. 95 In both cases, Hooke gave an alternative
explanation, thus demonstrating the superiority of his theory of colors over
Descartes’. These explanations were based on his own pulse theory of light,
according to which the short, vibrating motions of luminous objects produce
pulses that propagate rectilinearly through a transparent, homogenous
medium. 96
When such a light pulse falls obliquely on the surface of a denser
medium, the following happens. 97 Adopting Descartes’ viewpoint, Hooke
assumed that the pulses propagate faster in the denser medium. 98 The end of
the pulse that first reaches the surface will therefore come to move ahead of
the other end. As a result, the pulse will become oblique to its direction of
propagation, the refracted ray as found by means of the sine law. According
to Hooke the preceding end of the pulse is resisted most by the medium and
thus becomes weaker than its other end, whose passage has been prepared
by the first. This difference accounts for the primary colors red and blue.
With this ‘hypothesis’ Hooke explains the production of colors when light
passes a drop of water in a succession of refraction, reflection and another
refraction. 99 It may be clear that in this account a pulse is necessarily a
coherent whole, otherwise no difference can be made between its acute and
obtuse ends. Moreover, Hooke made no effort to mathematize the
mechanism presumed in the passage of the pulse to the denser medium, nor
did he suggest a macroscopic phenomenon comparable to it.
94 Hooke, Micrographia, 54.
95 Hooke, Micrographia, 59.
96 Hooke, Micrographia, 54-56.
97 Hooke, Micrographia, 56-59.
98 Compare Shapiro, “Kinematic optics”, 194-196.
99 Hooke, Micrographia, 61-62.