All our Concord waters have two colors at least; one when viewed at a distance, and another, more proper, close at hand. The first depends more on the light, and follows the sky. In clear weather, in summer, they appear blue at a little distance, especially if agitated, and at a great distance all appear alike. In stormy weather they are sometimes of a dark slate-color. The sea, however, is said to be blue one day and green another without any perceptible change in the atmosphere. I have seen our river, when, the landscape being covered with snow, both water and ice were almost as green as grass. Some consider blue "to be the color of pure water, whether liquid or solid." But, looking directly down into our waters from a boat, they are seen to be of very different colors. Walden is blue at one time and green at another, even from the same point of view. Lying between the earth and the heavens, it partakes of the color of both. Viewed from a hilltop it reflects the color of the sky; but near at hand it is of a yellowish tint next the shore where you can see the sand, then a light green, which gradually deepens to a uniform dark green in the body of the pond. In some lights, viewed even from a hilltop, it is of a vivid green next the shore. Some have referred this to the reflection of the verdure; but it is equally green there against the railroad sandbank, and in the spring, before the leaves are expanded, and it may be simply the result of the prevailing blue mixed with the yellow of the sand. Such is the color of its iris. This is that portion, also, where in the spring, the ice being warmed by the heat of the sun reflected from the bottom, and also transmitted through the earth, melts first and forms a narrow canal about the still frozen middle. Like the rest of our waters, when much agitated, in clear weather, so that the surface of the waves may reflect the sky at the right angle, or because there is more light mixed with it, it appears at a little distance of a darker blue than the sky itself; and at such a time, being on its surface, and looking with divided vision, so as to see the reflection, I have discerned a matchless and indescribable light blue, such as watered or changeable silks and sword blades suggest, more cerulean than the sky itself, alternating with the original dark green on the opposite sides of the waves, which last appeared but muddy in comparison. It is a vitreous greenish blue, as I remember it, like those patches of the winter sky seen through cloud vistas in the west before sundown. Yet a single glass of its water held up to the light is as colorless as an equal quantity of air. It is well known that a large plate of glass will have a green tint, owing, as the makers say, to its "body," but a small piece of the same will be colorless. How large a body of Walden water would be required to reflect a green tint I have never proved. The water of our river is black or a very dark brown to one looking directly down on it, and, like that of most ponds, imparts to the body of one bathing in it a yellowish tinge; but this water is of such crystalline purity that the body of the bather appears of an alabaster whiteness, still more unnatural, which, as the limbs are magnified and distorted withal, produces a monstrous effect, making fit studies for a Michael Angelo.
梭羅在此用了一大段文字描寫華爾騰湖的湖水顏色。他還用玻璃的顏色來作比較。但是我們現在知道這可能是兩回事。應該是無色的厚玻璃之所以會呈現綠色，跟它的材質(原料配方)有關；低鐵玻璃可以作到完全透明。而湖水和海水的顏色之所以會產生不同的變化，主要是因為大海的顏色是由海面反射的光和來自海水內部的回散射光的顏色決定的。湖水亦然。
太陽光線雖然從表面上看是白色的，可實際上它是由紅、橙、黃、綠、青、藍、紫七種可見光組成。這七種光線波長各不相同，而不同深度的海水會吸收不同波長的光，也就是說，海水對不同波長光的吸收和散射是有選擇性的。一般情況下，紅色和黃色等色光的波長比較長，最容易被海水吸收，當他們射入海水後，大部分光會被海水吸收，只有極少部分被水分子及海水中的懸浮顆粒反射和散射。而波長較短的藍光和綠光的穿透能力強，當它們射入海水後，只有少部分被海水吸收，大部分光遇到水分子或其他懸浮顆粒便向四周反射和散射，這樣一來，海水對藍光吸收得少而反射得多，而且越往海水深處越有更多的藍光被折回到水面上來。當這些被反射的藍色光射入到我們眼睛裏時，我們看到的大海便是藍藍的一片。 （問號小博士）
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