Through Magic Glasses and Other Lectures / A Sequel to The Fairyland of Science

by Arabella B. Buckley

"But the camera has at least one weak point. The screen at the back is not curved like our retina, but must be flat because of printing off the pictures, and therefore the parts of the photograph near the edge are a little out of proportion.

"In many ways, however, this photographic eye is a more faithful observer than our own, and helps us to make more accurate pictures. For instance, instantaneous photographs have been taken of a galloping horse, and we find that the movements are very different from what we thought we saw with our eye, because our retina does not throw off one impression after another quickly enough to be quite certain we see each curve truly in succession. Again, the photograph of a face gives minute curves and lines, lights and shadows, far more perfectly than even the best artist can see them, and when the picture is magnified we see more and more details which escaped us before.

"But it is especially when attached to the microscope or the telescope that the photographic apparatus tells us such marvellous secrets; giving us, for instance, an accurate picture of the most minute water-animal quite invisible to the naked eye, so that when we enlarge the photograph any one can see the beautiful markings, the finest fibre, or the tiniest granule; or affording us accurate pictures, such as the one at p. 19 of the face of the moon, and bringing stars into view which we cannot otherwise see even with the strongest telescope.

"Our own eye has many weaknesses. For example, when we look through the telescope at the sky we can only fix our attention on one part at once, and afterwards on another; and the picture which we see in this way, bit by bit, we must draw as best we can. But if we put a sensitive photographic plate into the telescope just at the point (i, i, Fig. 18), where the image of the sky is focused, this plate gives attention, so to speak, to the whole picture at once, and registers every point exactly as it is; and this picture can be kept and enlarged so that every detail can be seen.

"Then, again, if we look at faint stars, they do not grow any brighter as we look. Each ray sends its message to the brain, and that is all; we cannot heap them up in our eye, and, indeed, after a time we see less, because our nerves grow tired. But on a photographic plate in a telescope, each ray in its turn does a little work upon the chemicals, and the longer the plate remains, the stronger the picture becomes. When wet plates were used they could not be left long, but since dry plates have been invented, with a film of chemically prepared gelatine, they can be left for hours in the telescope, which is kept by clockwork accurately opposite to the same objects. In this way thousands of faint stars, which we cannot see with the strongest telescope, creep into view as their feeble rays work over and over again on the same spot; and, as the brighter stars as well as the faint ones are all the time making their impression stronger, when the plate comes out each one appears in its proper strength. On the other hand, very bright objects often become blurred by a long exposure, so that we have sometimes to sacrifice the clearness of a bright object in order to print faint objects clearly.

"We now come to our last magic glass—the Spectroscope; and the hour has slipped by so fast that I have very little time left to speak of it. But this matters less as we have studied it before.[2] I need now only remind you of some of the facts. You will remember that when we passed sunlight through a three-sided piece of glass called a prism, we broke up a ray of white light into a line of beautiful colours gradually passing from red, through orange, yellow, green, blue, and indigo, to violet, and that these follow in the same order as we see them in the rainbow or in the thin film of a soap-bubble. By various experiments we proved that these colours are separated from each other because the many waves which make up white light are of different sizes, so that because the waves, of red light are slow and heavy, they lag behind when bent in the three-sided glass, while the rapid violet waves are bent more out of their road and run to the farther end of the line, the other colours ranging themselves between."

Fig. 20.

Kirchhoff's spectroscope.

A, The telescope which receives the ray of light through the slit in O.