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But, unhappily, in proportion to the increase of size in the lens, and consequently of space-penetrating power, is the diminution of light from the object, by diffusion of its rays. And for this evil there is no remedy within human ability; for an object is seen by means of that light alone which proceeds from itself, whether direct or reflected.
Lister had begun his studies of the lens in 1824, but it was not until 1830 that he contributed to the Royal Society the famous paper detailing his theories and experiments.
The thousands of cards bearing the pictures at the outer ends are placed in a box, so that when the wheel of pictures is turned, by means of a crank attached to the axle, a projection holds each card in turn before the lens through which the observer looks. The projection in the top of the box acts like the thumb turning the pages of a book.
This double motion insures every part of the lens coming into contact with every part of the polisher, and moving over it in every direction. Then whatever parts either of the lens or of the polisher may be too high to form a spherical surface will be gradually worn down, thus securing the perfect sphericity of both.
With what jaundiced eyes we behold things! I myself saw all that was only through the lens of this diamond, of which not one of these men had ever heard.
Except the classical, this is the oldest written literature in Europe; and I doubt there is any other that gives us such a wide peep-hole into lost antiquity. Yes; perhaps it is the best lens extant, west of India. It is a lens, of course, that distorts: the long past is shown through a temperament, made into poetry and romance; not left bare scientific history.
At the upper end of the shaft is a mirror lens. Upon this mirror lens is reflected the surrounding surface of the ocean. The image reflected there is carried down the tube to other lenses and then conveyed to enlarging binoculars. Now do you understand?" "Perfectly," replied Jack; "and now as to the manner in which a submarine fights. It is by torpedoes, as I understand it."
If you lack a simple working theory, here is one now, in primer terms: The closer the object which you wish to photograph is to your lens, the more light it requires; the farther away it is, the less light it requires. This may sound somewhat unreasonable, but that is how a camera works.
I hastened home to feast my eyes once more on the lovely form of my sylph. I felt that henceforth to combat this passion would be impossible. I applied my eye to the lens. Animula was there, but what could have happened? Some terrible change seemed to have taken place during my absence. Some secret grief seemed to cloud the lovely features of her I gazed upon.
But at twelve o'clock MacWilliams lowered the glass quickly, with a little gasp of excitement, rubbed its moist lens on the inside of his coat and turned it again toward a limp strip of bunting that was crawling slowly up the halyards of the semaphore. A second dripping rag answered it from the semaphore in front of the Custom-House, and MacWilliams laughed nervously and shut the glass.
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