Meaning of “index to” in this paragraph

Question

In the following paragraph, what does the last sentence mean?

Accounts like these are sometimes copied into English and American journals. They lead the reader to infer that the arts and customs of civilized life are rapidly refining the natives of the Sandwich Islands. But let no one be deceived by these accounts. The chiefs swagger about in gold lace and broadcloth, while the great mass of the common people are nearly as primitive in their appearance as in the days of Cook. In the progress of events at these islands, the two classes are receding from each other: the chiefs are daily becoming more luxurious and extravagant in their style of living, and the common people more and more destitute of the necessaries and decencies of life. But the end to which both will arrive at last will be the same: the one are fast destroying themselves by sensual indulgences, and the other are fast being destroyed by a complication of disorders, and the want of wholesome food. The resources of the domineering chiefs are wrung from the starving serfs, and every additional bauble with which they bedeck themselves is purchased by the sufferings of their bondsmen; so that the measure of gew-gaw refinement attained by the chiefs is only an index to the actual state of degradation in which the greater portion of the population lie grovelling.

— Typee by Hermann Melville

I don't understand the “an index to the actual...” part. What does it mean?

Answer 1

The "index to" sentence is saying that the more wealthy the chiefs become, the more wretched the serfs are.

You could say that the wealth of the chiefs and the poverty of the serfs are inversely proportional.

Two definitions of the word index are

• an indicator, sign, or measure of something.
• something (as a physical feature or a mode of expression) that leads one to a particular fact or conclusion

So one can measure the poverty of the poor by measuring the wealth of the rich. Also, the opposite is true: you can measure the wealth of the rich by observing the poverty of the poor.

I have never seen this phrase before, it's very interesting used in this way.