In a well known Maxwell paper he uses the units of wavelength which he calls the Fraunhofer Measure. Which he gives 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:

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

But what is the motivation for this?