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The third mirror and peep hole is for back sights.
You turn back to the Sun and turn the octant 180 degrees about its vertical axis.
Then you can take the Sun altitude looking through this second visor, so you can see the horizon
opposite to the Sun, and touch it with the Sun's  reflection in the index mirror and the third mirror.

Uses: a) when the horizon under the sun is obscured by something (for example by a near shore).
b) to eliminate the dip and refraction. (If the sum altitude is h and combined dip and refraction is c>0,
then with direct sight you measure d=h+c, while with the back sight you measure b=180-h+c.
Thus h=(d-b)/2, and the correction for the dip and refraction cancels.

Alex.
 
 
________________________________________
From: NavList@fer3.com [NavList@fer3.com] on behalf of Chris Caswell [NoReply_Caswell@fer3.com]
Sent: Monday, March 11, 2019 11:46 PM
To: eremenko@math.purdue.edu
Subject: [NavList] Ebony Octant

Greetings All,

I was recently gifted an ebony and ivory octant, marked "Blachford & Imray, 
London", and I am wondering about the 3rd mirror (slitted) and peep sight, as 
I have never seen this set up before.

Does anyone know (or want to guess) what it is for?

Best,

Chris 44� 30' N 73� 15' W

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