Glass plates are heavy and inconvenient to carry, so that celluloid films have almost entirely taken their place, at least for outdoor work.

123.  Light and Shade.  Let us apply the above process to a real photograph.  Suppose we wish to take the photograph of a man sitting in a chair in his library.  If the man wore a gray coat, a black tie, and a white collar, these details must be faithfully represented in the photograph.  How can the almost innumerable lights and shades be produced on the plate?

The white collar would send through the lens the most light to the sensitive plate; hence the silver chloride on the plate would be most changed at the place where the lens formed an image of the collar.  The gray coat would not send to the lens so much light as the white collar, hence the silver chloride would be less affected by the light from the coat than by that from the collar, and at the place where the lens produced an image of the coat the silver chloride would not be changed so much as where the collar image is.  The light from the face would produce a still different effect, since the light from the face is stronger than the light from the gray coat, but less than that from a white collar.  The face in the image would show less changed silver chloride than the collar, but more than the coat, because the face is lighter than the coat, but not so light as the collar.  Finally, the silver chloride would be least affected by the dark tie.  The wall paper in the background would affect the plate according to the brightness of the light which fell directly upon it and which reflected to the camera.  When such a plate has been developed and fixed, as described in Section 121, we have the so-called negative (Fig. 83).  The collar is very dark, the black tie and gray coat white, and the white tidy very dark.

[Illustration:  FIG. 83.—­A negative.]

The lighter the object, such as tidy or collar, the more salt is changed, or, in other words, the greater the portion of the silver salt that is affected, and hence the darker the stain on the plate at that particular spot.  The plate shows all gradations of intensity—­the tidy is dark, the black tie is light.  The photograph is true as far as position, form, and expression are concerned, but the actual intensities are just reversed.  How this plate can be transformed into a photograph true in every detail will be seen in the following Section.

124.  The Perfect Photograph.  Bright objects, such as the sky or a white waist, change much of the silver chloride, and hence appear dark on the negative.  Dark objects, such as furniture or a black coat, change little of the chloride, and hence appear light on the negative.  To obtain a true photograph, the negative is placed on a piece of sensitive photographic paper, or paper coated with a silver salt in the same manner as the plate and films.  The combination is exposed to the light.  The dark portions of the negative