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 obtained by placing the spherules in position against the various radiant fields * which are furnished on Plate I.

4. Experiments on the production and effacement of outlines.

Experiment 1. Taking hold of an uncoloured glass spherule by its capillary stem, place it in position against a radiant field constituted by a white disc disposed on a black background. (Plate I, Fig. A).

Screen off so far as possible any light which may be incident on the upper surface of the object.

Note that, when the spherule is held at an appropriate distance (say 4-6 inches) from the illuminating field, it will appear as represented in the inset figure as a bright object delineated by a dark outline on a bright background.

Observe also that the breadth of the dark outline diminishes and increases as the spherule is brought nearer to, or, as the case may be, is carried away from, the radiant field.

Experiment 2. Substituting now for the radiant field just employed a bright field enveloping a black disc such as is provided in Plate I, B, and again bringing the spherule into position at a suitable distance from the central disc, observe that the sphere is now, as in the inset figure, delineated by a bright outline upon a black background.

Experiment 3. Substituting again for the radiant field employed in Experiment 2 a homogeneous white field such as is provided in Plate I, C, notice that the object is no longer delimited by any conspicuous outline. It will now, apart from reflections from its surface, be almost indistinguishable from the background.

Experiment 4. Substituting for the black and white fields employed in Experiments 1-3 a parti-coloured illuminating field consisting of a blue disc mounted centrally on a red field, as provided in Plate I, D, place the object in position at a suitable distance from this, and note that it appears, as shown in the inset figure, as a blue object delineated upon a blue background by a red outline.

Experiment 5. Place the sphere in position against any brightly coloured illuminating field (e.g. a blue field or a red field), hold it some little distance away, and shade off, as you do so, from its surface the colourless light which may otherwise fall upon it, and be reflected from it to the eye. Note that the spherule is now seen as a blue or red element delineated by a dark margin on a field of k blue or red (Plate Jif^Figs. / and g).

Experiment 6. Now allow a light from a window to fall upon the sphere, and note that the outline, instead of being black, is now, as is the case when Figs. / and g are viewed through the covering tissue paper, tinted with the colour complementary to the colour of the illuminating field, i.e., as the case may be with yellow and green.

1 These may be prepared for class instruction purposes by punching out, by means of a cork cutter, discs from white, black, and coloured paper respec- tively, and mounting these on appropriate backgrounds.