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Report of the Medical Officer (Education).
161
of schools as a whole. If separate class-rooms were taken as units, or if the separate place of eachchild was
considered in reference to its lighting, this high percentage of satisfactory illumination would no longer
be recorded. Until a couple of years ago the means of rapidly and accurately estimating local
illumination was scarcely available for school purposes. Now several methods exist for measuring
either (i.) the relative natural illumination of different parts of a room, which is the determination
usually wanted ; or (ii.) the absolute amount of illumination in foot candles on any surface, which is the
measurement required for estimating the value of artificial illumination, or for scientific investigations.
Before these convenient methods were available, it was recognised that apart from direct sunshine the
natural lighting of parts of a room was always a definite fraction of the out-door lighting. To
determine this relation the angular opening of the illuminating area of sky was measured. Recognising
that reflections in the room play an important part in its illumination, this method is being abandoned.
A better way is to use a portable arrangement by which the illumination on a small white surface in
any position can be compared against an image of the sky, the brightness of which can be diminished
to equality by means of the aperture of an iris diaphragm. This aperture in size measures the relative
illumination of the spot and includes the effect of all indirect and reflected light. The last method is
to measure the illumination in actual foot candles. A variety of handy instruments have been devised
recently. Mr. Harman has used a candle photometer made in a very portable and cheap form. Dr.
Williams uses an electric apparatus in the schools, and Dow's Lumeter is a reliable instrument which
has recently become well known. With these means at disposal school illumination will now enter on
a new phase. The lighting at any place can be stated as a fraction of daylight, or in definite foot
candles.
The physiological adjustment of the eye is such that it can bear an illumination by daylight
which would be the cause of unmitigated glare by artificial light. Outside illumination is usually many
hundred times as great as indoor natural lighting. It is generally stated as a thousand times greater
than that of an average room. For this reason a relative photometer, such as that of Thorner, already
mentioned, which measures by the aperture of an iris diaphragm the relative value of the illumination
at various parts of a room in relation to the outside lighting, is the most convenient routine method
of recording the natural lighting of a class room. The amount of light varies enormously in different
school places. Figures 12 and 13 give measurements made by Dr. Williams in the absence of the
GROUND FLOOR ROOM E
WRITING (FLAT) DESKS
children at two o'clock on a clear, bright, but sunless day in November in two wide class rooms of the same
school—the top of the windows being 13 feet high. The light is shown to vary ten or twelve fold on
the desks. Desks which are stippled in the diagrams do not exceed 2 foot candles, and are judged too
dark for school work. The minimum illumination of a school place should be 2.5 foot candles. On
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Figure 12.—Measurements of desk lighting in a schoolroom (lowest floor). Stippled desks are too dark for
school work.