From: Ed Popko
Date: 2018 Jul 14, 04:12 -0700
Thinking about Frank's analogy of how we read an analog clock's hand position, the Astra IIIB sextant offers yet another example where you have to read between the marks.
The Astra's vernier is marked in minutes and additional steps of two-tenths of a minute. However, you can interpolate down to the tenth of a minute even though there are not single miute marks.
The attached images show a setting of 65 degrees, 10.5 minutes. Although there is not .5 minute mark on the tenths scale, the marks for .4 and .6 are symetrically aligned between two whole minute marks so use it as if it was .5