DIOPTRICS
, called also anaclastics, is the doctrine of refracted vision; or that part of Optics which explains the effects of light as refracted by passing through different mediums, as air, water, glass, &c, and especially lenses.
Dioptrics is one of the most useful and pleasant of all the human sciences; bringing the remotest objects near hand, enlarging the smallest objects so as to shew their minute parts, and even giving sight to the blind; and all this by the simple means of the attractive power in glass and water, causing the rays of light in their passage through them to alter their course according to the different substances of the medium; whence it happens, that the object seen through them, do, in appearance, alter their magnitude, distance, and situation.
The ancients have treated of direct and reflected vision; but what we have of refracted vision, is very imperfect. J. Baptista Porta wrote a treatise on refraction, in 9 books, but without any great improvement. Kepler was the first who succeeded in any great degree, on this subject; having demonstrated the properties of spherical lenses very accurately, in a treatise first published anno 1611. After Kepler, Galileo gave somewhat of this doctrine in his Letters; as also an Examination of the Preface of Johannes Pena upon Euclid's Optics, concerning the use of optics in astronomy. Des Cartes also wrote a treatise on Dioptrics, commonly annexed to his Principles of Philosophy, which is one of his best works; in which the true manner of vision is more distinctly explained than by any former writer, and in which is contained the true law of refraction, which was found out by Snell, though the name of the inventer is suppressed: here are also laid down the properties of elliptical and hyperbolical lenses, with the practice of grinding glasses. Dr. Barrow has treated on Dioptrics in a very elegant manner, though rather too briefly, in his Optical Lectures, read at Cambridge. There are also Huygens's Dioptrics, an excellent work of its kind. Molyneux's Dioptrics, a work rather heavy and dull. Hartsoeker's Essai de Dioptrique. Cherubin's Dioptrique Oculaire, et La Vision Parfaite. David Gregory's Elements of Dioptrics. Traber's Nervus Opticus. Zahn's Oculus Artificiah Teledioptricus. Dr. Smith's Optics, a complete work of its kind. Wolsius's Dioptrics, contained in his Elementa Matheseos Universalis. But over all, the Treatise on Optics, and the Optical Lectures of Newton, in whose experiments are contained far more discoveries than in all the former writers. Lastly, this science was perfected by Dollond's discovery of the acromatic glasses, by which the colours are obviated in refracting telescopes.
The laws of Dioptrics see delivered under the article Refraction, Lens, &c; and the application of it in the construction of telescopes, miscroscopes, and other | dioptrical instruments, under the articles Telescope, and Microscope.
DIP of the Horizon. See Depression.
Dipping-Needle, or Inclinatory Needle, a magnetical needle, so hung, as that, instead of playing horizontally, and pointing out north and south, one end dips, or inclines to the horizon, and the other points to a certain degree of elevation above it.
The inventor of the Dipping-needle was one Robert Norman, a compass-maker at Ratcliffe, about the year 1580; this is not only testified by his own account, in his New Attractive, but also by Dr. Gilbert, Mr. William Burrowes, Mr. Henry Bond, and other writers of that time, or soon after it. The occasion of the discovery he himself relates, viz, that it being his custom to finish, and hang the needles of his compasses, before he touched them, he always found that, immediately after the touch, the north point would dip or decline downward, pointing in a direction under the horizon; so that, to balance the needle again, he was always forced to put a piece of wax on the south end, as a counterpoise. The constancy of this effect led him, at length, to observe the precise quantity of the dip, or to measure the greatest angle which the needle would make with the horizon. This, in the year 1576, he found at London was 71° 50′. It is not quite certain whether the dip varies, as well as the horizontal direction, in the same place. Mr. Graham made a great many experiments with the dipping-needle in 1723, and found the dip between 74 and 75 degrees. Mr. Nairne, in 1772, found it somewhat above 72°. And by many observations made since that time at the Royal Society, the medium quantity is 72°1/2. The trifling difference between the first observations of Norman, and the last of Mr. Nairne and the Royal Society, lead to the opinion that the dip is unalterable; and yet it may be difficult to account for the great difference between these and Mr. Graham's numbers, considering the well-known accuracy of that ingenious gentleman. Philos. Trans. vol. 45, pa. 279, vol. 62, pa. 476. vol. 69, 70, 71.
It is certain however, from many experiments and observations, that the dip is different in different latitudes, and that it increases in going northward. It appears from a table of observations made with a marine dipping needle of Mr. Nairne's, in a voyage towards the north pole, in 1773, that
| in latitude | 60° | 18′ | the Dip was | 75° | 0′, | |
| in latitude | 70 | 45 | the Dip was | 77 | 52, | |
| in latitude | 80 | 12 | the Dip was | 81 | 52, | and |
| in latitude | 80 | 27 | the Dip was | 82 | 2 1/2. |
Burrowes, Gilbert, Ridley, Bond, &c, endeavoured to apply this discovery of the dip to the finding of the latitude; and Bond, going still farther, first of any proposed finding the longitude by it; but for want of observations and experiments, he could not go any length. Mr. Whiston, being furnished with the farther observations of colonel Windham, Dr. Halley, Mr Pound, Mr. Cunningham, M. Noel, M. Feuille, and his own, made great improvements in the doctrine and use of the dipping-needle, brought it to more certain rules, and endeavoured in good earnest to find the longitude by it<*> For this purpose, he observes, 1st, That the true tendency of the north or south end of every magnetic needle, is not to that point of the horizon, to which the horizontal needle points, but towards another, directly under it, in the same vertical, and in different degrees under it, in different ages, and at different places. 2dly, That the power by which the horizontal needle is governed, and all our navigation usually directed, it is proved is only one quarter of the power by which the dippingneedle is moved; which should render the latter far the more effectual and accurate instrument. 3dly, That a dipping-needle of a foot long will plainly shew an alteration of the angle of inclination, in these parts of the world, in half a quarter of a degree, or 7 1/2 geographical miles; and a needle of 4 feet, in 2 or 3 miles; i. e. supposing these distances taken along, or near a meridian. 4thly, A dipping-needle, 4 feet long, in these parts of the world, will shew an equal alteration along a parallel, as another of a foot long will shew along a meridian; i. e. that will, with equal exactness, shew the longitude, as this the latitude.
This depends on the position of the lines of equal dip, in these parts of the world, which it is found do lie about 14 or 15 degrees from the parallels. Hence he argues, that as we can have needles of 5, 6, 7, 8, or more feet long, which will move with strength sufficient for exact observation; and since microscopes may be applied for viewing the smallest divisions of degrees on the limb of the instrument, it is evident that the longitude at land may thus be found to less than 4 miles.
And as there have been many observations made at sea with the same instrument by Noel, Feuille, &c, which have determined the dip usually within a degree, sometimes within 1/2 or 1/3 of a degree, and this with small needles, of 5 or 6, or at the most 9 inches long; it is inferred, that the longitude may be found, even at sea, to less than half a quarter of a degree. This premised, the observation itself follows.
To find the Longitude or Latitude by the DippingNeedle.—If the lines of equal dip, below the horizon, be drawn on maps, or sea-charts, from good observations, it will be easy, from the longitude known, to find the latitude; and from the latitude known, to find the longitude either at sea or land.
Suppose, for example, a person travelling or sailing along the meridian of London, should find that the angle of dip, with a needle of one foot, was 75°; the chart will shew that this meridian, and the line of dip, meet in the latitude of 53° 11′; which is therefore the latitude sought.
Or if he be travelling or sailing along the parallel of London, i. e. in 51° 31 north latitude, and find the angle of dip 74°; then this parallel, and the line of this dip, will meet on the map in 1° 46′ of east longitude from London; which therefore is the longitude sought.
