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How to measure Index Correction using the Sun by Starparth, is a widely read paper describing a useful technique for determining a sextant's Index Error and thus, it's Index Correction. The introduction states ...

This is an old method, developed by those explorers who did most sights on land using an artificial horizon (Lewis and Clark, for example)." It's a simple and very useful technique for today’s celestial navigator and I have used it many times. 

However, on page 2, the write-up states:

(6) Now take the difference between the ON value and the OFF value and divide that by 2 to find your IC. Just subtract the smaller from the larger. The label of your result will be the same as the label of the larger value. In this example: 34.0 - 30.8 = 3.2’ and 3.2’/2 = 1.6’ and since 34 was ON, the answer is ON, ie our IC is 1.6’ ON the scale.

Why is the stated result Index Correction and not the sextant's Index Error.  I'm confused. This reversal would affect the interpretation of the examples given in the supplementary pages at the end as well.  

Perhaps IE and IC have not had a standard meaning.  I have always associated Instrument Error - IE - with the sextant (the instrument)  and IC with how to correct it.

Ed