The stimulation of the retina does not pass off immediately the cause ceases to operate. Its effect lasts for a distinct period, about the ith of a second. If, therefore, two impressions follow one another at a less interval they appear as continuous, since the effect of the one has not passed off before the other is produced. If, in a series of flashes, one follows another at less than the interval named, the impression of a continuous flash will be produced. It is thus that a string, glowing red-hot at one end, and rapidly whirled round, produces the impres sion of a circle of light. This fact is taken advantage of in the construction of the wheel of life. Here a set of pictures is produced on a circular band of paper, which is set in a revolv ing wheel. The pictures represent a man, let us say, in the different positions he would be in, one instant after another, during the act of walking, for example. One picture follows another in its proper order, and when the wheel is rapidly revolved the appearance of the man walking is produced.
The Perception of the retina the complete terminal apparatus of vision all that one could be conscious of would be a sensation of light whenever the retina was stimulated, but we could have no definite know ledge of the object from which the light pro ceeded. Photographers obtain a picture of a person by the use of a plate of glass on which is a film sensitive to light. This sensitive plate is placed in a box or camera, facing the per son. But were the camera a simple box with a hole in front through which the light could fall on the plate behind, the result would be a uniform darkening of the film from the ex posure to light and no picture would be pro duced. What the photographer desires is to throw on the plate an image of the person in light and shade. The parts of the sensitive film on which the light portions of the image fall are strongly acted on, and the parts on which the shadows fall are feebly acted on, and more or less feebly as the shadows are deep or slight. The sensitive plate is thus unequally acted on, and when the photographer has submitted it to the action of certain chemical solutions the film is left thick and dark where the strong light fell, but thinner and more or less transparent in the places corresponding to the shadows. If then he holds his plate up to the light and looks through it, he sees in light and shadow an image of the person who sat before the camera. But to obtain this there must be certain definite parts of the sensitive plate corresponding to certain parts of the person. Thus if the light
is shining strongly on one side of the person's face, the sensitive plate must receive the rays reflected from that part of the face, and these rays must not be diffused over the whole plate, but made to fall on a part of the plate corre sponding accurately in outline and in propor tionate size to the part from which they have proceeded. So it must be with the rest of the figure. On the plate there must be parts corre sponding to the parts of the person to be pho tographed. It is the same in vision. If not merely a general impression of light is to be obtained, but a definite knowledge of things, then on the retina there must be distinct lumi nous impressions, distinct regions of light and shadow corresponding to the lights and shadows of the object from which rays of light are pro ceeding to the eye. In short, we cannot see in absolute darkness, we see only when light enters the eye, and we see definite things only when rays of light fall on them and are by them reflected into the eye. If all objects reflected light equally from their whole surface we could not see things defined from one another; and we would have simply a consciousness of a uniformly illuminated surface. Things have definite out lines, and forma, because light is unequally reflected from different parts of their surface, the illuminated object being mapp,d out by the shadow that surrounds it. It is only, then, when such illumination and shadow are accurately reproduced on a sensitive photographic plate that an image of a person or object is obtained, and only when accurately reproduced on the sensi tive coat of the eye that we can see things distinctly. How then is this accurate re production of light and shadow obtained 1 Let us examine the photographer's appa ratus, for in it is an accurate representa tion of the eye.
The photographic camera is a box (MN, Fig. 177), the inner surface of which is painted a dull black, and which is light-tight. In front is an opening into which is screwed a brass tube (An) fitted with a series of convex lenses, shown in the upper part of the figure (a L). A screw (v) enables the tube containing the lenses to be worked backwards or forwards in an outer case. The box is closed at the back by a-ground-glass plate (o), capable of being removed. No light enters the box except through the opening in front (which may be closed by a cap), and it must pass through the lenses on its way.