The general term is analogy.
The A : B :: C : D format is known as the Aristotelian format:
If you took the SAT test sometime before 2005, you are no doubt
familiar with Analogy Questions in the Aristotelian format, SKY : BLUE
:: GRASS : _. (Sky is to Blue as Grass is to what?) [tresna, DashingBean]
In ancient Greek the word αναλογια (analogia) originally meant
proportionality, in the mathematical sense, and it was indeed
sometimes translated to Latin as proportio. From there analogy was
understood as identity of relation between any two ordered pairs,
whether of mathematical nature or not. Kant's Critique of Judgment
held to this notion. Kant argued that there can be exactly the same
relation between two completely different objects. The same notion of
analogy was used in the US-based SAT tests, that included "analogy
questions" in the form "A is to B as C is to what?" For example, "Hand
is to palm as foot is to ____?" These questions were usually given in
the Aristotelian format:
HAND : PALM : : FOOT : ____
While most competent English speakers will
immediately give the right answer to the analogy question (sole), it
is more difficult to identify and describe the exact relation that
holds both between hand and palm, and between foot and sole[citation
needed][original research?]. This relation is not apparent in some
lexical definitions of palm and sole, where the former is defined as
the inner surface of the hand, and the latter as the underside of the
foot. Analogy and abstraction are different cognitive processes, and
analogy is often an easier one.
It's important to note that the above analogy is not comparing all the
properties between a hand and a foot, but rather comparing the
relationship between a hand and its palm to a foot and its sole.
While a hand and a foot have many dissimilarities, the analogy is
focusing on their similarity in having an inner surface.
lovanda at Wordreference.com adds:
Grammatically A is to B as C is to D and A is to B what C is to D
are both correct.
There is a subtlety involved here in that the 'to' in 'facts are to the scientist' carries increased semantic weight. Beyond the 'relates to' sense (eg of 50 : 100) is the 'are, in the arsenal ...' sense (ie 'facts are the bread and butter of the scientist').