In every perception two factors contribute to the result. The one is the nature of the object perceived, the other that of the percipient. The effect of the first factor is obvious and well recognized; the importance of the second factor is more apt to be overlooked. The sunset is a different experience to the artist from what it is to the farmer; a piece of rocky scenery is viewed with quite a different interest by the artist and the geologist. The things that were attractive in childhood have lost their charm; and what was then, if noticed at all, considered stupid, has become a cherished hobby. Even from day to day, our interests change with our moods, and our views of things brighten with the weather or the good behavior of our digestive organs. Not only will the nature of the impression change with the interests of the observer, but even more, whether or not an object will be perceived at all, will depend upon the same cause. The naturalist sees what the stroller entirely overlooks; the sailor detects a ship in the distant horizon where the landsman sees nothing; and this is not because the naturalist and the sailor have keener vision, but because they know what to look for. Whenever an impression is vague, or an observation made under poor conditions, this subjective element comes to the front. Darkness, fear, any strong emotion, any difficulty in perception reveal the same influence. "La nuit tous les chats sont gris." Expectation, or expectant attention, is doubtless the most influential of all such factors. When awaiting a friend, any indistinct noise is readily converted into the rumbling of carriage-wheels; the mother hears in every sound the cry of her sick child. After viewing an object through a magnifying-glass, we detect details with the naked eye which escaped our vision before. In spite of the fact that the answer in the book happens to be wrong, a considerable proportion of the class succeeds in reaching it. Everywhere we are apt to perceive what we expect to perceive, in the perception of which we have an interest. The process that we term "sensation," the gathering of evidence by the senses, is dual in character, and depends upon the eyes that see as well as upon the things that are present to be seen.