CHAPTER VII

FORM--_Continued_. LIGHT ECONOMIZED BY RIGHTLY SHAPED GLASS. HEAT SAVED BY WELL DESIGNED CONVEYORS AND RADIATORS

Why rough glass may be better than smooth . . . Light is directed in useful paths by prisms . . . The magic of total reflection is turned to account . . . Holophane globes . . . Prisms in binocular glasses . . . Lens grinding . . . Radiation of heat promoted or prevented at will.

A Shrewd Observer Improves Windows.

These are times when an inheritance, such as the window pane, venerable though it be, is freely criticized and shown to be far from perfect. We find, indeed, that surfaces and forms long given to the glass through which light passes, or from which light is reflected, are faulty and wasteful. This means that sunshine can be turned to better account than ever before, that artificial light can be employed with an economy wholly new. A few years ago when we provided a window with plate glass, smooth enough for a mirror, nothing better seemed possible. Thanks to the late Edward Atkinson, of Boston, we know to-day that in many cases glass may be too smooth to give us the best service, that often we may get much more light from panes of rough, cheap make than from costly plate glass. He tells us: “In 1883, when I inspected a large number of English cotton mills, I found them glazed with rough glass of rather poor quality, the common glass of England being inferior to our own from the general lack of good sand. On asking why rough glass was used instead of smooth I was told that rough glass gave a uniform and better light. To my astonishment I found this true. The interior of a mill so lighted had the aspect of diffused illumination. This led me to reason on the subject. I looked into the construction of the Fresnel lens, in which a combination of lenses and curved surfaces concentrates rays of light into a single far reaching beam. I reasoned that if one set of angles or curves could thus concentrate light, then by reversal of such angles or surfaces, light could be diffused.”

Mr. Atkinson proceeded to gather specimens of glass not only of common rough surface, but also in ribbed and prismatic forms. These he handed for examination and comparison to Professor Charles L. Norton of the Massachusetts Institute of Technology, Boston. His report says: “The hopelessness of trying to get something for nothing, that is, to get a sheet of window glass to throw into a room more light than fell upon it, appeared so plain to me that I made all my preparations to measure not a gain but a loss of light in using Mr. Atkinson’s samples. The results of the tests may be briefly stated: In a room thirty feet or more deep we may increase the light to from three to fifteen times its present effect by using ‘Factory Ribbed’ glass instead of plane glass in the upper sash. By using prisms we may, under certain conditions, increase the effective light to fifty times its present strength. The gain in effective light on substituting ribbed glass or prisms for plane glass is much greater when the sky-angle is small, as in the case of windows opening upon light shafts or narrow alleys. With the use of prisms a desk fifty feet from a window has been better lighted than when but twenty feet from the same window fitted with plane glass. . . . ‘Ribbed’ and ‘Maze’ glass are of very great value in softening the light, especially when windows are directly exposed to the sun, aside from their effectiveness in strengthening the light at distant points. With the ‘Maze’ glass the artist may have, in all weathers and in all directions, what is in effect a much-desired north light. The same glass provides the photographer with light as well diffused as when cloth screens or shades are employed and of much greater intensity.”

Plate prism glass is now manufactured with its outer or street surface ground and polished like plate glass, with its prisms accurate and smooth. In dimensions which may reach fifty-four by sixty inches it affords surfaces easily kept clean, and transmitting much more light than glass held in frames of small divisions.

Whence the gain in thus exchanging plane glass for glass rough, ribbed, or prismatic? Rays streaming through an ordinary window strike nearby surfaces of wall, ceiling, and floor; from these they are reflected in large measure and return through the glass to outer space. Rough, ribbed, or prismatic glass throws the rays much further into the room, hence they strike so much larger an area of wall, ceiling, and floor that in being reflected again and again the light is well diffused, and but little is sent forth again into outside space. The form of the glass gives the entering light its most useful direction, so that the new panes serve better than the old. This effect is most striking when prisms are carefully adapted to a particular case in both their angles and their placing. In traversing glass, light is absorbed and wasted, so that the shorter its path the better. In the compound lens devised in 1822 for lighthouses by Augustin Jean Fresnel, light is as effectively bent by the part of the glass shown in dark lines as if the whole lens were employed.

This brings us to means for the best use of artificial light. Within the past thirty years the standard of illumination, thanks to electricity, has steadily risen. More important than ever, therefore, is it that light should be employed pleasantly and effectively. This in the main is a question of placing the sources of light judiciously, and of so reflecting and refracting their rays that they will be of agreeable quality, and arrive where they are wanted with the least possible loss. Reflectors rightly shaped and kept clean economize much light. For lack of them in streets and squares we may sometimes observe half the rays from a lamp taking their way to the sky where they do no good. In shop windows ribbed reflectors throw full illumination on the wares displayed, while the sources of light are out of view. The same method is employed in art galleries and in museums. A parabolic reflector sends forth as parallel rays the powerful beam of a lighthouse, a locomotive, or a searchlight. An incandescent lamp of ingenious design is silvered on its upper half so that none of its light is wasted. Because the arc lamp is the cheapest of all illuminants it is adopted for out-of-door lighting where its unpleasant glare is tempered by distance. In factory lighting its brightness is excessive and harmful unless moderated. A capital plan is to employ an ordinary continuous current and place the positive carbon, with its brilliant centre, below the negative carbon; beneath these two carbons a good reflector throws the rays to the ceiling, whence they descend with agreeable diffusion and much less loss than when globes of ground glass surround the arc. A common white ceiling when quite flat is an excellent reflector; indeed, a sheet of white blotting paper returns light nearly as well as a polished mirror, and for many purposes it serves better; the mirror sends back its beam in a sharply defined area which may be dazzling, the paper scatters light with thorough and agreeable effect.

Usually a mirror is a sheet of highly polished metal, or a plate of glass with a quicksilver backing; preferable to either is clear glass, all by itself, so formed as totally to reflect an impinging beam of light. To understand the principle involved in its use we will for a little while bid good-by to lamps of all kinds.

Delight and Gain as We Watch a Fish in Water.

A hall of delights is the New York Aquarium, in the historic Castle Garden at the Battery. Its tanks display a varied and superb collection of fish, whose beauty of form and color heightened by swift and graceful motion, fascinates the eye as no museum of dead things, however splendid, ever does. When a tank is still, or nearly still, and a gold-fish or a perch is quietly resting near the surface of the water, one may see its form reflected from that surface as perfectly as if by a mirror. The point of view must be close to the tank, with the eye somewhat lower than the fish. So perfect, at times, is this mirroring that young folks are apt to suppose the reflection to be a second fish, and they are puzzled to remark how strangely it resembles its mate just below. What explains this reflection? A ray of light can always pass from a rare medium, such as air, into a dense medium, such as water, because it is bent toward their common perpendicular. But a ray cannot always pass from a dense into a rare medium, from, let us say, water into air, for if the ray were to be bent away from the common perpendicular more than 90° it would altogether fail to emerge from the water. No luminous ray can pass from water into air if it makes a greater angle with the perpendicular than 48° 35´. Suppose AB (page 78) to be the water level of a tank. A ray leaving F will be bent so as to reach C, a ray from G will reach D, a ray from H will reach E; but a ray from L will be bent so much as to pass along the surface of the water as OB, and a ray from I will be bent so as to return beneath the surface of the water to I. Rays such as I, undergoing total reflection, afford us our second image of a fish at rest near the surface of water: to observe this kind of image we need not journey to the New York Aquarium; with patience we may behold it in a small home aquarium with flat sides of clear glass, waiting until the water is quiet and a fish comes close to the surface.

Every dense transparent substance has this ability to yield images by total reflection, each substance having a critical angle of its own; we have just seen that for water this angle is 48° 35´. Glass is made in many varieties, each with a special critical angle, never much different from that of water. A right-angled prism of glass, which any optician can supply, serves as a capital mirror for rays striking its surface at ninety degrees. Such prisms are employed in opera glasses, in hand telescopes, in reflectors for light-houses, and in the Holophane globes we are about to examine. The efficiency of these prisms may be as much as 92 per cent., whereas that of the best silvered mirrors never exceeds 90 per cent. The loss in a prism is due to a slight reflection by the surface on which the rays first fall, and by the absorption of light in the glass itself; this second loss, of course, increases with the thickness of the prism.

Total Reflection in Artificial Lighting: Holophane Globes.

Now that we understand the principle of total reflection, let us see how it is applied to increasing the effectiveness of a Welsbach mantle or an electric lamp. And first let us say that we may wish light upon a small area, mainly in a single direction, as downward upon a desk or reading-chair. Or, in a quite different manner, if we are to illuminate a wide space such as that of a large parlor. These requirements are fulfilled by the Holophane globes, devised by M. Blondel and M. Psaroudaki, which are made in many shapes, each adapted to a specific duty. The upper half of each globe is formed into prisms of such angles that, zone by zone, the glass totally reflects impinging rays in just the directions desired. The contouring is accurate to the thousandth part of an inch. With this thorough reflection is combined diffusion as thorough, the interior of the globe being shaped as ribs. Thus, with the least possible waste, the upper half of the source of light is utilized. What of the lower half? Its rays pass through prisms formed so as to refract impinging light into desired paths with but little loss. As a whole, therefore, these globes furnish a beautiful means of illumination with all but perfect economy, special forms of them sending light in any direction desired.

Total Reflection in Binocular Glasses.

In the Zeiss Works at Jena, in Germany, optical instruments of the highest excellence are manufactured; many of these take advantage of the principle of total reflection we have been considering. When the task was assumed of producing a new and improved telescope, it was observed that an ordinary telescope, built up of lenses, is inconveniently long and heavy in comparison with its magnifying power. The question arose whether it was possible to construct short instruments of a magnifying power of four to twelve diameters. Porro, an Italian, about the middle of the nineteenth century suggested totally reflecting prisms so placed that while the total travel of a ray would be the same as in an ordinary telescope, the two ends of the luminous path would be near together, while the whole would be more effective than if four mirrors were employed. His idea may be represented by a wire one meter long so bent that its ends are much less than one meter apart. In an illustration of a field-glass as manufactured at the Zeiss Works, on the Porro principle, it will be remarked that the entering ray passes through lenses which are farther apart than the lenses which form the eye-pieces. Thus a much wider field is viewed than that of an ordinary glass, while as the two images received from the two eye-pieces differ more than those observed in direct vision, the perception of depth is increased in a notable degree. This construction is adapted to sporting, marine, and opera-glasses, as well as to field-glasses.

Lenses Still Much Used.

Lenses nevertheless continue to be much more important than prisms, and the proper shaping of their surfaces involves high reaches of both science and art. The properties of the glass, of course, count for most in producing combinations free from color for telescopes, microscopes, and cameras. Jena glass, described in another chapter of this book, with its extraordinary range of refractive and dispersive qualities has brought optical instruments to virtual perfection. Meanwhile the arts of lens-grinding leave little to be expected in the way of future improvement. It is astonishing that a lens forty-two inches wide can be so truly curved as to focus the image of a star as an immeasurable dot.

The Production of Optical Surfaces.

Let us look at some of the instruments designed by a master for shaping glass discs into lenses. Some of the best telescopes in existence are from the hands of Mr. John A. Brashear, of Allegheny, Pennsylvania. The grinding tools he employs he has contoured in such wise as to produce desired curves free from error. The first polishers are of the ordinary form with square or circular facets equally distributed over the surface of the tool, as in Figs. H and 8. When the polish is brought to its best, the glass is allowed to cool slowly to a normal temperature, and is then carefully studied as to its defects. These are removed and the surfaces finished with iron tools, of the same diameter as the surface to be worked, each tool being laid off into six sections, as in Figs. 3, 4, 5, 6, 7. The tool being warmed, pitch is spread over its leaf-like spaces, which are given the proper curve by being pressed down on the previously wetted concave surface; the pitch and tool are next quickly cooled with water. In the shaping of these spaces rests success. The zone, a, a, in the first figure, needing the greatest amount of abrasion, meets the widest part of the leaflet, but in order that no zonal error may be introduced, as in b, c, c, b, of the second figure, it is gently tapered in each direction, the amount of taper being governed by the lateral stroke given to the polisher, as well as by the amount of departure of the zone from the normal curve.

But after all the astronomers aided by lenses thus carefully shaped are few, while millions of people suffer from defects of sight which are overcome by suitably formed spectacles.

Bi-focal Spectacles.

In this field a recent minor improvement is worthy of mention. Benjamin Franklin many years ago made a pair of spectacles in which the upper half of each glass was ground for far seeing, the lower half for near seeing. To-day such bi-focal spectacles are not made in halves, with an unpleasant broken line across them. In each of the new eyeglasses toward the base a small lens of dense quality is enclosed; through this lens a wearer looks at objects nearby; through the upper part of the eyeglass he looks at distant objects. The joining of the three parts is effected so skilfully as not to be discernible.

Economy of Heat.

From light we pass to its twin phase of energy, heat, for a glance at the forms of devices which enable us to use heat with economy. When we wish a furnace, crucible, or cooking vessel to maintain the highest possible temperature, we give it as little surface as possible. On the contrary when a warming apparatus is devised, its surface is freely extended. The traditional fireplace, for all its cheerfulness, yields but little heat. Benjamin Franklin copied its form in the stove which bears his name; as it stands out from a wall it warms the air all around itself, instead of on one side only. This model is familiar in gas stoves, whose heat thoroughly radiated and convected far exceeds that derived from fireplaces. In Canada forty years ago it was usual, especially in the country, to set up gallows-pipes and dumb-stoves, or drums, bulky, hollow structures of sheet iron, which obliged the heated products of combustion to take a roundabout course as they passed to the chimney. To be sure as thus cooled the gases were less effective as draft makers, but we must remember that one of the most wasteful uses of fire is in warming air or other gases for the sake of putting them in motion. In modern factories, central lighting stations, and the like huge installations, mechanical draft sends a quick current through a short chimney, saving much fuel. Excellent in design are the tile stoves of Germany and Holland. Their gentle heat does not parch the air; in moderately cold weather they render it unnecessary to light furnaces which develop, at such times, unduly high temperatures.

In factories the heating coils filled with steam or hot water were at first fastened to the floor. Then came attaching them to the ceiling whence their heat is gently radiated; on the floor the coils may gather dust and dirt with risk of fire; with the other plan there is a saving of floor space, and accidental leaks are at once in evidence.

Tubes for warming are specially effective when dented or buckled in directions at right angles to each other and to the axis of the tube. This form gives the heating water or steam a swirling motion which causes it to part more rapidly with its heat than does a cylindrical tube of the same surface. Gold’s electric heater for street-cars, bath-rooms, and the like, is a spiral of resistant alloy, hung in a light metallic frame, the whole presenting a large surface to the air. Automobiles driven by heat engines require coils of the utmost possible surface whereat cooling can take place; in many cases this cooling is furthered by the action of a quick fan. In like manner the condensers of steam-engines, especially aboard ship, are made up of slender tubes presenting to the steam a chilling area of vast extent.

Inventors have long addressed themselves to the difficulty caused by the expansion and contraction of structures as temperatures change. For years the cylindrical fire-boxes of marine boilers have been corrugated, so as to allow them a certain play without breaking from their fastenings, or tearing their seams, when heated or cooled. This form is adopted with success for the Morison fire-boxes of the Vanderbilt locomotives. In quite different situations metal piping, in a length of let us say 100 feet, is provided against trouble from shrinkage or expansion by a U bend. When the diameter of the pipe is twelve inches, this bend is usually about ten feet in extent; for a six inch pipe, a bend six feet long suffices. Another difficulty due to heat is the limitation of speed imposed by the heat which friction creates. A new type of circular saw has a hollow arbor through which flows cold water, so that motion may be faster than ever before. The same arbor appears in various other machines with like advantage.