The Fraunhofer Measure

Question

In a well known Maxwell paper he uses the units of wavelength which he calls the Fraunhofer Measure. He states it for the Fraunhofer D and F bands as

$$\lambda_D = 2175 \text{ crazy units} = 589nm$$

$$\lambda_F = 1794 \text{ crazy units} = 486nm$$

So the conversion is:

$$1nm \approx 3.69\text{ crazy units}$$ $$1 \text{ crazy unit} \approx 0.270nm$$

But what is the motivation for this?

I think the original conversion factor in the question was wrong; I've made an edit fixing it. — Jason Davies, Commented Nov 24, 2012 at 21:26
The "crazy unit" is probably a characteristic lattice length scale (between neighboring atoms in a lattice) for a common material, which was easy to compare diffraction experiments with. So all materials were measured relative to this "standard candle" material. — Qmechanic, Commented Nov 24, 2012 at 21:59
@Qmechanic I think the reference in my answer below clears it up; the Paris inch was probably more common in those days! — Jason Davies, Commented Nov 24, 2012 at 22:06

Jason Davies · Accepted Answer · 2012-11-24 21:40:38Z

2

I'm guessing this is related to the archaic Paris inch, which is $27.069$mm, i.e. $10^8 \times$ the conversion factor.

answered Nov 24, 2012 at 21:09

Jason Davies

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$\begingroup$ It's from exactly the same area of literature, so it makes sense. Oh, and it says K is Mrs Maxwell (J is Mr) $\endgroup$
– Lucas
Commented Nov 24, 2012 at 23:20

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