NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: "Vernier acuity" of horizon IC tests
From: Bill Morris
Date: 2009 Jul 6, 15:08 -0700
From: Bill Morris
Date: 2009 Jul 6, 15:08 -0700
George, Frank wrote: ?If you remove the telescope from a sextant and hold it (the sextant) at arm's length pointing at the horizon, the human visual system (eye+visual cortex) is able to detect remarkably small deviations in the visual line of the horizon.? and you responded ?I don't understand where such a difference arises, with telescope discarded, between holding the instrument at arm's length and holding it close to the eye? Close to the eye the line between the silvered and unsilvered protions of the horizon mirror is out of focus. At arms length, it is better focussed, as in, say, the foresight of a rifle. A sighting tube, by increasing depth of focus by means of the eye stop, has the same effect. Frank also wrote: ?By contrast, when a telescope is attached or when the instrument is held close to the eye, the horizon on the direct side of the field of view fades away slowly and merges with the reflected view on the other side of the field of view. We align these horizon images by superimposing them. This is not a hyperacuity task, and so the results are limited by normal resolution.? and you responded: ?It seems logical to me, that if a 7x telescope makes no improvement in the eye's ability to align two images of a horizon, that's because either the telescope is a very bad one, or, more likely, that the horizon is an unsharp one (in which case, magnification will make it no sharper).? A x 7 telescope is likely to be a Keplerian rather than a Galilean. Each half of the former ?sees? the whole of the field and Frank is quite correct in saying that there is a wide area of overlap of the two horizon images. Each half of a Galilean ?sees? its own half of the field, but there is still a narrow central area of overlap. Using either type of telescope, we do not align the ends of two lines. Rather, there is a fairly sudden appearance of increased sharpness as the two images merge. This is quite different from what happens when we align the ends of two lines. If we accept the reality of vernier acuity, and I think we must, we would expect vernier acuity using the naked eye to be at least as good as merging images using a telescope, as Frank has postulated earlier; and this is what an appeal to experiment might show. Greg has done an experiment (post 8967), but I feel his numbers in each series are rather small and may not have sufficient statistical power. If no one else does it first, I will repeat the experiment when I have got over a stiff neck from calibrating a series of sextant micrometers. Bill Morris Pukenui New Zealand --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---