We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
Human Eyes and Optical Instruments III — Spectroscopes
EARLY in the construction of telescopes it was found that a simple lens brings different colors to a focus at different distances. Most objects radiate or reflect several colors. Consequently, if light of one color is in focus that of other colors is out of focus and the result is very unsatisfactory. In fact, Newton despaired of making satisfactory refracting telescopes and turned his attention exclusively to reflectors.
Two hundred years ago the unequal refraction of light of various colors was lamented by scientific men. It was supposed that this property of light seriously impaired optical instruments without offering any compensating advantages. Doubtless many supposed that if they had had the privilege of establishing the laws of Nature they would have avoided all such difficulties. Fortunately men have not made the laws of Nature. They might have made the refraction of all :olors the same and thus have simplified the construction of telescopes, but in doing it they would have made impossible the spectroscope, an instrument as marvelous as any we possess and one which penetrates fields that were supposed to be inaccessible.
Figure 1.
The principle underlying the spectroscope is simply that when light passes through a refracting medium, such as a glass prism, the different colors are refracted, or bent, different amounts. Figure 1 illustrates the principle, though the indicated arrangement is not adapted to practical work. A beam of light L strikes the screen S through which there is a narrow slit o. A thin sheet of light passes through o and falls on the glass prism P. All the rays are bent downward, but, of those which are visible tohuman eyes, the blue are bent the most and the red the least. On emerging from P to a rarer medium the rays are bent down still more, again the blue being bent the most and the red the least. They fall on the screen M, each color in its own position.
11