CHAPTER XXXVI.—THE CAMERA OBSCURA: HOW TO MAKE AND USE IT.
By Gordon Stables, C.M., M.D., R.N.

Did you never, reader, have a peep in beneath the black cloth where the artist hides his head while he is focusing a sitter for his photograph? I’m sure that many of you have. And what did you see? Why, a pretty little picture in colours of your friend sitting in the chair, laughing like a tramp at a twopenny roll, only upside down. And you have said to yourselves, What a pity it won’t come out in bright colours like that, and why in all the world should it be upside down?

Now I will answer this question before going any further, because it has a bearing on the subject before us—the making of a handy and cheap camera obscura, which cannot fail to be a source of amusement and pleasure, especially when the sun shines.

The reason why the object on the photographer’s ground-glass plate is seen upside down is easily explained. Light, as I need hardly remind older boys, proceeds in straight lines from any illuminated object. It is thus thrown upon the photographer’s plate. A glance at the accompanying diagram ([Fig. 1]) will suffice to show what I mean.

Fig. 1.

Let A B be the object—say, an arrow—under consideration, and C D a side view of the ground-glass plate on which the picture is seen. Passing, therefore, in straight lines, the light and colour from the point A will fall at a, will they not? and those from B at b, and so on from every portion of the arrow, so that the representation therefore on the object-glass will be upside down, or reversed. Q.E.D.

Now about the camera. No one can be said to have invented it, for it is constituted upon the firm and immutable laws of Nature. Roger Bacon is credited with having known this principle. Very likely he did, but he put it to no practical use, though over four hundred years after his time Giovanni Baptiste Porta did. But who knows that the ancients hundreds of years before the Flood were unacquainted with it? Here, for example, is a story a little bird told me one beautiful summer’s day while reclining on the greensward in my woodland study: I had been reading under the shade of my great oak-tree. The sun was very bright, and patches of its light penetrated even through the dark-green branches and fell on my face. Probably it was that which set me a-thinking about the laws of optics and the camera obscura and camera lucida. Suddenly close up above me a bird alighted—it was early in the season—and began pouring out the most charming notes.