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Prism Paper
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December 1, 1900 THE SCHOOL JOURNAL. 567

Light Diffusing Media.

No subject is exciting keener interest among architects just at present than the use of the various forms of prismatic, ribbed, and maze glass in securing a more effective use of natural and artificial light. The importance of this matter in schools can hardly be overestimated. One of the first conditions for effective and healthful work is plenty of light. In most schools the students near the windows suffer from too strong light while those at a distance sit in comparative darkness. Very much of the nearsightedness as well as the extreme nervousness of our school children grows out of habits of reading or study in badly lighted rooms at home or at school.

An exterior installation [of prism glass]
An Exterior Installation.

The Atkinson-Norton Experiments.

An account of important experiments in regard to the diffusion of light performed by Mr. C. L. Norton at the Massachusetts Institute for Technology was printed in THE SCHOOL JOURNAL for Nov. 3.

Commercial Applications of Prismatic Glass.

Of the four forms of glass tested in the Atkinson-Norton experiments the prism glass seems to be the most satisfactory. Whether the so-called "factory-ribbed" glass which showed up so well in the tests will immediately be commercially available remains to be seen.

Luxfer Prisms.

The products of the American Luxfer Prism Company, of Chicago and New York, have for several years been available for purpose of school lighting. They have already been installed in a great many educational institutions. An installation which Mr. A. P. O'Brien, manager of the New York office, happened to be figuring on when a SCHOOL JOURNAL representative visited him was in the Albany, N.Y., high school. Other schools in the neighborhood of New York which already have the Luxfer prisms are as follows:
St. Mary's school, 8 East Forty-Sixth street; Trinity church school, 70 Church street; public school No. 42, Allen street, near Hester street; school, 168 West 79th street; Morristown high school, Morristown, N.J. The Ethical Culture school, 129 West 34th street.

What Luxfer Prisms Are.

Luxfer prisms are section of crystal glass of a standard dimension of four inches square, having a smooth outer surface and an inner surface divided into a series of small accurately formed prisms. This size has been found by experience to be most convenient, both for the manufacturers and for the building architects. It is also artistically best suited for the type of window that prevails in the modern buildings of to-day.
Luxfer Prism

There are a great many varieties of prism used, so that the Luxfer company finds it advisable for one of their own engineers to study the problem of each installation where that is possible. Each case requires some special treatment, being dependent not only upon the size and shape of rooms and windows, but upon the surroundings of the building, and the direction from which light comes to the window. One of the strongest points made by this company is that they study each problem of lighting in a scientific fashion much as an expert oculist studies the problems of getting light to the retina of the eye in the best possible fashion. The commonest Luxfer arrangement provides for a lower sash, glazed with plane glass and an upper sash of prism glass, the size and quality of this varying greatly according to the nature of the interior to be lighted.
Most of the big department stores are now saving on their electric light bills by introducing this form of glass. The New York, Philadelphia, Boston, and Chicago stores are already well equipped with it.
One other point that is made by the Luxfer people is worth noting. Their glass does not merely perform the function of a light transmitter; it also becomes part of the architectural decoration of the building. It is a known fact among architects that ordinary window glass does not enter properly into artistic harmony with the other elements, but is rather a necessary evil. Hence the extensive use of stained glass in buildings where an aesthetic effect is sought. The Luxfer prisms when properly applied, even to the most expensive and ornate buildings, are found to be a highly ornamental feature in the entire façade. When looked at from the outside, they do not have the appearance of glass at all. This is, of course, a consideration in the erection of modern school-houses in which use and beauty go side by side.

The Diffusion of Artificial Light.

Hardly less important to the school than the correct distribution of daylight is the question of the diffusion of artificial light. Wherever there are night classes, in libraries and reading rooms, in physical laboratories and studios, the problem of getting a light that is physiologically harmless and usable educationally must be faced.
It is well known that students in evening classes are especially liable to afflictions of the eyes. Library work, too, even under the best conditions is very trying in the evening. Anyone who has sat face to face with an incandescent light in a college library for four hours is likely to know how essential it is that the new discoveries in the way of diffusion of light shall be applied to interior illumination.
As a general statement it may be said that progress in the last few years has been in the direction of producing light sources of great intensity. The arc and incandescent electric lights were a step in this direction. When [Carl] Auer von Welsbach made his remarkable discovery of a means for utilizing the hitherto unconsumed elements in gaslight, thereby increasing its efficiency seven or eight times, he added another powerful illuminator to the list. Very lately acetylene gas has been found to be capable of producing an intense white light that almost vies with daylight in brilliancy. All these new forms of illumination are powerful and, unmodified, full of physiological danger.

What Good Illumination Requires.

Primarily there should be a sufficient amount of light thrown upon the objects to render them easily visible; and in looking at the objects the eye should not be dazzled by intense rays direct from the source itself.

The Holophanes.

The first attempt on a large scale to make a commercial application of the principles of diffused light to artificial illumination is now being made by the Holophane Company, of New York. Holophane lamp cover This firm has adopted the form of prismatic glass that was worked out by Blondel and Psarondaki, two Parisian scientists, in strict accordance with optical laws. What they claim for the Holophane glass is that it embodies the following requisites in lighting:
  1. Diffusion of light (softening or removal of glare), equaled only by "opal" glass and giving double the amount of illumination.
  2. Downward deflection of upward rays, combining reflection with diffusion.
  3. Downward deflection of rays near the horizontal that cannot be reached by any other system of reflectors.
  4. Distributing the rays of any particular source of light in the spaces where they are most needed, thus greatly increasing the effective illumination.
  5. Covering the light source so that neither the form, number of lights, nor the fixtures are visible, the glass itself becoming the practical source of light
  6. Rich appearance.
  7. A method of manufacture which insures the scientific accuracy of an optical instrument in a cheap form of glass.
The Holophane globes are made of transparent glass, so that none of the light rays are intercepted. The inner surface of the glass is given over to carefully calculated flutings or prisms used solely for diffusing or softening the light without loss of power. On the outside face are prisms calculated for reflecting these diffused rays into directions where needed.
In practice, Holophane glass, when placed over a light, will render a dazzling light soft and healthful, while increasing its effective illuminating power.

The "Luminous Prisms."

Another firm which has begun to do some work in the way of school installations is the New York Prism Company, of 473 West Broadway, New York. Their business thus far has been principally with private schools, tho they have had some orders from the boards of education of Philadelphia, Cleveland, and Chicago. In New York city they have supplied prismatic glass to the following institutions: The Normal college; the Brearley school; the Berkley school; the Cornell medical college; the Chapin Collegiate school.
Revolving prism canopy
This prism company has recently printed "Natural Light for School-Rooms," an essay addressed to the National Educational Association, at Charleston. In this essay the facts about prismatic lighting are very succinctly and forcibly stated. It should be in the hands of every student of school hygiene in the country.
This firms employs a lucical engineer who makes scientific studies preparatory to each installation.

Maze Glass as a Light Diffuser.

In the Atkinson-Norton tests the ordinary maze glass which is very commonly used in airshafts, transoms, and other openings where a soft diffused light is desired made a remarkably strong showing. In especial the maze made by the Mississippi Glass Company, of St. Louis and New York, came in for a very high place among the light diffusers. Two of the officials of the company, when interviewed by THE SCHOOL JOURNAL expressed themselves as not only gratified but very much surprised by the scientific prominence that had been given to their products, for they have never undertaken the manufacture of light diffusers upon scientific principles. Their varieties of maze glass are simply made for the satisfaction of the insurance companies who demand a glass that shall be valuable in the fireproofing of buildings. Maze glass, with wire ribbing, suits this requirement ideally and is sold in great quantities. The illustration here shown is of the variety of maze glass which stood highest in the recent tests.
The Mississippi Glass Company has not yet seriously considered the problems of school lighting. They are glad, however, to have the effectiveness of their mazes known. It may be stated that the maze glass is peculiarly adapted to the lighting of rooms which are not very deep. A glazing of the upper sashes of all the windows of an ordinary school-room lighted from two sides with ordinary maze glass would result in a very soft, even light throughout the room. For studies, too, it is the ideal glazing. If an entire south window is glazed with maze glass, the light will be as uniform as in a studio with north exposure.
The Mississippi Glass Company people attribute much of their success in the technology experiments to the quality of their glass which is made from the sand of the Mississippi valley, generally conceded to be the finest in the world.
Diffusion pattern from prism globe
This illustration indicates, on the left-hand side, the diffusion of light thru a prism globe; on the right-hand side, the diffusion thru a globe of ordinary plate glass. The downward direction of the rays from the Holophane should be noted; the light is thrown exactly where it is most needed.