Light's speed is c: in a vacuum, in deep space.

(Approximately.)

Boyer saves some of his most emphatic points in
"The Rainbow:..." for the closing pages:

"The structure of vibrating atoms is vastly more
complicated than the few simple motions proposed
by Huygens, Newton, Young, Fresnel, and Maxwell.
In 1887, the year before Hertz discovered his waves,
Stokes wrote of the wave theory as ''a thing at the
present day resting on evidence quite overwhelming''.
Today the situation has changed so radically that as
to occasion the facetious remark that physicists accept
the wave theory three days a week and hold to the
corpuscular doctrine three days a week, and every
seventh day they humbly admit their lack of understanding."

"[Boitel] concluded that ''the theory of Airy seems to
need to be completed.'' Moreover, through very carefully
conducted experiments of his own, Boitel obtained results
not entirely consistent with those from the Airy integral,
and he suggested that ''the theory of Airy is therefore but
a first approximation.'' How familiar a situation this is
in the history of science, and especially in the story of
the rainbow! What one generation hails as a ''complete
theory'', the next relegates to the status of a ''first
approximation''."

"Pernter took particular pains to point out the inadequacy
of the Cartesian explanation and to clarify the theory of
Airy. As he pointed out, the phrase ''Cartesian effective
ray'' is misleading. It is not the only effective ray; and, indeed,
Airy had shown that among all the effective rays, it is not
even the most efficacious. The phrase ''least deviated'' is
the correct designation for DesCartes' ray."

"Pernter reiterated that the geometrical [corpuscular,
rectilinear] theory cannot account for any of these
variations, and ''inasmuch as it only holds for drops of
infinite radius, it must be completely abandoned.'' While
Pernter's extreme position is of course correct, in a strict
sense, the Cartesian theory has remained the backbone of
the schoolboy's explanation of the rainbow because it
affords so eminently clear and direct a first approximation."

"Aesthetes were grieved at Newton's simple materialistic
explanation of color. But if they could know of the intricate
maze of creative mathematics required for the study of the
rainbow, they would realize how misguided they are - or else
how thoroughly avenged! Within a raindrop the interaction
of light energy with matter is so intimate that one is led directly
to quantum mechanics and the theory of relativity."

"The corpuscular and wave theories of light seem to have
far more in common than Newton and Huygens had ever
dreamed, for in 1924 Louis de Broglie (b.1892) established
a synthesis of views in which light is made up of tiny entities
dragging trains of waves with them."

account for any of these variations

https://sopha2018.sciencesconf.org/187262/document" rel="nofollow" target="_blank">https://sopha2018.sciencesconf.org/187262/document
https://sopha2018.sciencesconf.org/187262/

"In the concluding remarks we pledge for a reconsideration
of infinitesimalism by sketching out some of the prospects
of working with absolute continua by uncovering a whole
range of implications of studying the structures introduced
by Veronese and those who followed in his way, interesting
for contemporary philosophy of mathematics, philosophy
of physics and even mathematics and physics proper."