While digging through the old literature on refraction, notably
Carl Christian Bruhns's
**historical essay**,
I found that the traditional series expansions for astronomical refraction
were introduced by
**J. H. Lambert**.

I had first encountered references to Lambert as a graduate student; his
famous *Photometria* was cited in works I read while learning
astronomical photometry.
And of course “Lambert's cosine law” is well known to physicists;
I probably had heard of it even as an undergraduate.

I soon learned that Lambert's “perfectly diffusing surface” that
embodies his cosine law is one of those idealizations familiar to
first-year physics students. Along with the frictionless track, the
inertialess pulley, and the massless string, it is an imaginary
idealization that has no counterpart in the real world. Indeed,
Arthur Searle commented a century ago [*Observatory* **22**,
310–311 (1899)] that

. . . Lambert's theory, developed in his `Photometria,'

had been regarded as almost demonstrably true, while, in fact, it

consisted of an ingenious mathematical superstructure on a very

insecure foundation.

In particular, it was already known by then that planetary surfaces did not obey Lambert's “law”. For example, both Zöllner and Müller in the 19th Century had established that the phase curves of Moon, Mercury, and Mars all differed substantially from that of a Lambertian sphere. Again, as a graduate student, I had read their works.

Copyright © 2003, 2005, 2006 Andrew T. Young

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