This post was preceded by “An Improvised Dip Meter”
In the preceding post, I wrote a little about the Blish prism and the Gavrisheff dip meter and pointed out that if the angle between two horizons opposite to each other could be measured, the local dip could be deduced. Through the kindness of Alex Eremenko, I have recently been able to examine in detail an N 5 Russian dip meter which has several interesting design features. It is probably easier to appreciate these if an idea is had of the optical path of the instrument (Figure 1). I explained a little about dip and its importance in celestial navigation in the preceding post.
Figure 1 : Light path of Russian N5 dip meter.
The horizon is viewed simultaneously to the left and right of the observer. Taking the rays from the left side first, shown in green, they pass through an adjustable slit, used to make the brightness of the two horizons equal, through a watertight window in the side of the instrument and thence to a roof prism, where they are deflected downwards at 90 degrees. They then pass through a semi-reflecting junction, and are again deflected by a second prism through 90 degrees in a plane at right angles to the first, into the objective of a x 4 Keplerian (inverting) telescope.
The rays from the right hand horizon also pass through a window and then through a weak positive (convex) achromatic lens of 1.5 dioptres power. They then enter a negative (concave) lens whose power is such that the two lenses together have no net power, so that they behave like a piece of plane glass. However, the positive lens can be moved up or down from a central position, so that the light path also moves up or down – but not by very much. In fact, the total travel of the lens is 12 mm and the total deflection of the rays is 15 minutes of an arc each way. The right rays continue on through the right angled prism and are reflected off the common, semi-reflective face, off the opposite face and thence into the telescope objective, where they are combined to be viewed through the eyepiece. A yellow filter can be attached to the latter to reduce glare. Figure 2 shows the practical realisation of Figure 1.
Figure 2 : Practical light path
Projecting from the bottom of the slide that carries the sliding lens is a boss which carries an adjustable cam follower. This cam follower is held by two anti-backlash springs against a large cam (Figure 3). The cam is rotated by means of the adjusting ring, which carries a scale graduated plus or minus 15 minutes from zero, each minute being subdivided to 0.2 minutes (Figure 4).
Figure 3 : Adjusting cam.
Figure 4 : 3/4 Plan view.
Figure 5 shows the 4-power telescope. The images of the two horizons are seen to be vertical and, by rotating the adjusting ring they are brought into coincidence, when the dip can be read off the scale. The zero is set using two autocollimators, aligned as described in the preceding post An Improvised Dip Meter.
Figure 5 : The telescope.
The instrument is well-sealed against the ingress of moisture by greased felt rings where there are rotating parts and by heavy wax at metal-to-metal and glass-to-metal joints. It is provided with a stout leather carrying case. Figure 6, modified from a drawing in the original Russian handbook, shows the general arrangement of the parts. It can be seen at a larger scale by clicking on the picture. Use the back arrow to return to the text.
Figure 6 : General arrangement drawing.
To use, the window with the slit is directed to the brighter horizon and the slit adjusted to make its brightness equal with the opposite horizon. The adjusting ring is rotated to bring the horizons (which will be seen to be vertical) into coincidence and the reading noted.. The observer then rotates through 180 degrees (about a vertical axis, of course) and also rotates the instrument through 180 degrees on a horizontal axis. A second reading is taken and the dip taken as the mean of the two.
I have provided this post to support my book The Nautical Sextant, which covers solely the nautical sextant. If you have enjoyed reading this and others of my posts, I am sure you will enjoy reading the book, available from the publishers, from Amazon and from good booksellers.
The Nautical Sextant 
Have a Russian naval dip meter with very few moving parts, only focus and iris slot have moving parts. It is set up exactly as fig 1 in yr article shows except there seems to be no way of making the pod and neg lenses if these are present, move. Removing the cover reveals a large prism or prisms which fill the available space. The prism contains a block with the layered glass as in fig 1, and the layers appear as a set of wide black parallel lines when looking into either side port. I will put photos on a webpage and post link here.
John
It sounds as though the grease in the adjusting ring (see Figs 4 and 6) has solidified and jammed the ring and/or cam. A “quick and dirty” solution might be to squirt some releasing compound into the area and leave it with the eyepiece uppermost in a warm place overnight. Better would be to do a proper overhaul, starting by removing the cover plate (six screws) and working inwards towards the cam.
Bill
Thanks, here’s link to page with photos. https://navaldipmeters.wordpress.com/2016/03/02/russian-fixed-lense-variant/
The two controls have heavily knurled finger surfaces, and only include the iris ring and focus ring marked in diopters +5 to -5. I can find no internal mechanism which would cause relative motion between two lenses, nor can I see such lenses.
John
In the centre right photo, the tubular portion which looks as though it has rubber knurling, looks as though it ought to rotate. At the right hand end of the tube is a small rectangular piece held in place by a screw. I think this is probably a stop, to limit its rotation. I think I can see a +5 in relation to an index on the tubular part in the upper left hand photo. Could it be that this is an indication of dip rather than an index for the dioptre ring of the eye piece? In your place I would carefully anatomise the tubular part and eyepiece to see what lies within. It plainly is a dip meter. The mystery is how dip is indicated.
Bill
Thanks. Looking thru the eyepiece with the two side-ports looking horizontal, there’s a vertical scale marked in 15 tics above and 15 below the zero mark in the vertical center. The scale has no numbers, just small horizontal lines for single units and longer lines every 5 units. The scale appears the same whether looking out one side port or the other. One very heavy, short mark which may be a cursor, is on the vertical scale at the 7th unit from zero, but if it is a cursor and not a loose black paint chip, I’ve found no way to move it. The two knurled lugs near the eyepiece do change the focus of the scale (reticle.). It seems that the long, pleated rubber turbe over the eyepiece tube is for gripping when turning the focus ring, as it takes force to do that. There isn’t any joint in the eyepiece tube that would indicate the middle of it could be twisted, and in fact it doesn’t twist.
There must be something that you are missing. If it takes force to turn the focus ring, something else must be stuck as well. If the tube won’t/can’t rotate, perhaps it slides. There will be something that you can alter until the horizons seen from each side align and you will then read the dip off the 0 to + and – 15 arc minutes scale.
Bill
Here’s some info on the inventor. Since I know Germany had optical dip meters ca. WWI, the Russian named didn’t invent the very first one, but apparently one version of the device. This model has been superceded by a later one in the Russian Navy. http://www.encyclopedia.com/doc/1G2-2830902263.html
I exhausted all possibilities for some mechanism by which to bring horizon images into coincidence and failed to find any. I thought an experiment might reveal how this 1953-calibrated model might work. At mid-day on a sunny day, I set up two opposite horizons (colored horizontal bars) both 42 inches above ground, one black and one silver-colored. The instrument was positioned at 46.5 inches above ground, midway between the two horizons each 15 feet distant. I followed the procedure taking readings in minutes by counting units on the reticle. On this model the horizons appear horizontal in the eyepiece. Readings were a bit difficult to take at first even with no ship motion or salt spray, but I got used to it. Readings were 9 minutes when I faced north and 10 minutes facing south after rotating instrument about the horizontal axis (horizons used were east and west of instrument.). So this device is simplistic compared to the later N5 shown in this article. I’ll post a photo taken thru eyepiece during the experiment.
Here’s a photo taken thru the eyepiece during the experiment. I couldn’t get the reticle any clearer for the camera although it focused perfectly clear for the eye. There’s much extraneous material in the photo which should be ignored.