NavList:

David Pike,

Regarding quintant photos at the link you provided in your 4 Nov 24 post, you wrote: “. . . there appears to be a way of making them read from 40 to 180 degrees by having an upper and a lower scale.  How do they manage that?”. . . and in your later post of 5 Nov 24 you wrote: “Has anyone any idea of the light paths and why anyone might be interested in measuring 180°?” 

My take on the design follows.

First off, nobody could measure 180 degrees with that design. It uses a positive vernier (0 to the right) that spans 15 degrees on the arc, so if the index arm could be swung far enough to put the left end of the vernier at the 180 degree graduation, the vernier index would be at 165 degrees.  (My assessment below shows it couldn’t measure as much as 165 degrees either.)

The first attachment to this post is a photo from your link on which I’ve marked angles that I measured. On it, the two red lines are horizontal reference markers, with the bottom one connecting like points at the extreme ends of the arc. The two turquoise lines are lines of sight through the upper and lower horizon mirrors. The upper line is depressed 5 degrees below horizontal and lower line is 20 degrees below the upper (25 degrees below horizontal).

The two yellow lines are parallel to the horizon mirrors, making angles of 69 degrees for the upper mirror and 49 degrees for the lower, referenced to the lower red horizontal marker. As the index mirror should be parallel to the horizon mirror when the index arm is set to 0, it, too, should be at 69 degrees when using the upper horizon mirror.

To use the upper horizon mirror, align the scope line of sight with it and tip the quintant up 5 degrees for a horizontal view. (See the second attachment to this post, which is the first attachment, rotated clockwise 5 degrees. A violet arrow marks the line of sight.) This increases the horizon mirror angle from 69 degrees to 74 degrees. The index mirror is parallel to it when the index arm is set to 0, so it, too, is now at 74 degrees. Rotating the index arm through 60 degrees (120 degrees on the arc) increases its angle to 134 degrees.  Assuming the index mirror angle must be at least 5 degrees greater than the angle to be measured (no reflection occurs when they are equal), this is satisfactory.

To use the lower horizon mirror align the scope line of sight with it and tip the quintant up 25 degrees for a horizontal view. (See the third attachment, which is the first attachment rotated clockwise 25 degrees. The line of sight is indicated by the violet arrow.) This increases the horizon mirror angle from 49 degrees to 74 degrees. The index mirror angle was 74 degrees when the line of sight was raised 5 degrees, but now it has been raised 20 degrees higher, to 94 degrees. The extra 20 degrees true is 40 degrees on the arc, so what was called 0 on the lower scale is 40 on the upper. Rotating the index arm through 60 degrees increases its angle to 154 degrees. With the above assumption of ‘index mirror angle needs to be at least 5 degrees greater than the angle to be measured’, this result would be good to 149 degrees.

What is potentially possible with this design, given my assessment of it?

It uses a positive vernier (0 to the right) that occupies 15 degrees on the arc. If the index arm could be swung far enough to put the left end of the vernier at 180 degrees on the upper scale and 140 degrees on the lower scale, the vernier index would be at 165 degrees on the upper scale and 125 degrees on the lower scale. These are the maximum angles the arc allows if the vernier is to be used. To use them with the above assumption of ‘index mirror angle 5 degrees greater than angle to be measured’ the index angle must be greater than 130 degrees when using the lower scale (which it is), and 170 degrees on the upper scale (which it isn’t).

Something’s wrong. Is it me?

I have 1897 and 1901 U.S. Navy specifications for surveying sextants with single horizon mirrors where the 1897 spec required ability to measure 130 degrees with an arc graduated from – 5 to 145 degrees, and the 1901 spec required a measurement capability of 145 degrees with an arc graduated from -5 to 160 degrees. Both specs required a positive vernier occupying 13 degrees on the arc. I have photos of a later version that is graduated to 180 degrees, but I don’t have the specification for that version. Based on the consistent difference of 15 degrees between the required measurement angle and upper graduation angle (to accommodate the 13 degree vernier extent) in the 1897 and 1901 specs, I’d expect the requirement for the later version was to measure to 165 degrees.

Chuck V.

File:

File:

File: