The rays of light may be separated by the prism into a band of five different colors--red, yellow, green, blue, and violet. Three of these colors--the red, yellow, and blue--are called the primary colors, for any color or shade desired may be obtained by blending and mixing them in different proportions. These three primary colors have each a separate power. The red rays possess heating power, the yellow rays possess illuminating power, and the blue rays chemical power. The blue rays are called actinic, and when we speak of actinic light we mean the blue rays which produce the chemical change in the sensitive plate. The effect of these actinic rays may be seen in other things besides the sensitive plates. The fading of carpets, draperies, and clothing, the tanning or browning of the skin, etc., are due to their action. After the sensitive plate has been exposed in the camera to the chemical action of the blue rays, the change which has taken place is invisible to the eye, and in this state is called the latent image, because it is dormant or hidden. In order to preserve this chemical change in the silver salts the sensitive plate must be washed or soaked in a solution which will form an opaque compound with the part of the salt which has been acted upon by the light. As it is necessary to have a light to watch the process and stop it when it has been carried far enough, we must have a light free from actinic or blue rays. We therefore darken the room and use a red light, for the red rays have little or no actinic power. As we watch the chemical change which takes place in the sensitive plate when covered with what we call the developer, we notice black patches appear here and there on the plate. These are the places which have been exposed to the strongest actinic rays. All bodies radiate or reflect light, some more than others. A piece of yellow silk may appear to the eye lighter in color than a piece of blue silk, but when the two pieces are photographed it will be found that the yellow photographs much darker than the blue silk. This is because the yellow silk does not reflect actinic rays, while the blue does, and therefore the sensitive plate is more strongly affected by the light reflected from the blue than from the yellow silk. The yellow-colored silk possesses the illuminating power which causes it to make a room look bright and sunny, while the blue silk possesses the chemical power which affects the sensitive salts.