It is also stated that every color of light has a definite wave length. Where a band of blue color appears upon the film we know that its thickness is right for the wave length of that particular color which is reflected from the back of the film to the eye. If we could conceive the blue vault of the heavens to be half a sphere of a soap bubble, the color that the sky would appear to us (if the light could be thrown upon it from beneath) would be determined by the thickness of this film. If the film was 1-156,000 of an inch the sky would be red instead of blue. To reflect the other colors the film would have to grow thinner for each color, in the progression from red to violet. The color of the sky is determined by a light-reflection from minute globules of moisture floating in the air. If the sky is blue, then the globules must be of the right diameter to reflect that color. The various tints and colorings of the sky are determined by what is found in the atmosphere, and this is the reason why skies differ in coloring and tone in different sections of the globe. The finest skies are probably found in semi-tropical regions like southern Italy, Greece, and California. In 1892 I visited Greece in the early part of June. In crossing the Adriatic, from Brindisi to Patras in Greece, the route was through the Ionian Islands that are grouped along the southwestern shore of Albania. The sky was without a cloud, and its beautiful blue color was reflected in the waters of the Adriatic, and I never shall forget the impression made upon my senses when we first came in sight of the mountains on the west coast of Albania. At this point they rise abruptly from the water and are colored with that peculiar azure haze, mixed with a shading of warmth, which is an effect that distance gives in the classic atmosphere of old Greece. The effect upon the beholder is to intoxicate the senses and to fill him with that deliciously poetic feeling that always comes when standing in the presence of the sublime in nature. It was not the mountains themselves that produced the effect, for I had seen grander than these; but it was the sky on the mountains. When we look at a distant mountain it seems to be partly hidden by a peculiar haze that is the color of the sky at that time; we are really looking at the mountain through a portion of the sky. While in Athens I took a trip to the top of Mount Pentelicus, which separates the plains of Athens on the south from those of Marathon on the north. From the summit of this mountain we have a most wonderful view of the archipelago of the Ægean Sea--a beautiful map of blue water and brown islands that melt together in the distance. At our feet lay the historic plains of Marathon, and in the distance rose the snow-capped peaks of Mount Olympus. It is doubtful if the world furnishes a more beautiful combination of ocean, island, continent, and sky than can be seen from Mount Pentelicus. Myriads of