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Page:Popular Science Monthly Volume 15.djvu/832

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angular position of the prism, with regard to the collimator, is a matter of importance, the distinctness and purity of the spectrum depending in a great measure on that position. Perhaps as good a way as any is to find by actual trial the angular position of the prism which gives the best results, thus: Focus the telescope for distant vision, align the telescope with the collimator, and move the slit till it (the slit) is seen distinctly in the telescope; then put the prism in place approximately, and move it round its axis until a position is found where (supposing the light of the sun or diffused daylight to be under examination) the lines in the green part of the spectrum are seen at their best advantage. It must be remembered that a slight change of focus is necessary for every color. When the bounding edges of the ribbon of variegated light which forms the spectrum are seen sharply defined at the same time that the lines across the spectrum are distinct, the adjustment is pretty correct; when such is not the case, the slit and lens are not in their proper relative positions. The narrower the slit, consistent with the necessary supply of light, the finer and more distinct are the lines. A black cloth, or a pasteboard box, with suitable apertures for the collimator and telescope tubes, should be placed over the prism to shut out all extraneous light. In conclusion, I must remark that I do not pretend to describe an instrument capable of doing exact or delicate work. My aim is simply to show with how little trouble or ingenuity a spectroscope may be put together which will help materially those students who wish to obtain a good general idea of a branch of science which has done more to unravel the mysteries of nature than any other inquiry with which the human mind has ever been occupied.



THE question of the source of muscular power is essentially a question concerning transformation of energy. The most characteristic distinction between plants and animals is, that the former appropriate force from outside themselves, from sunlight, and store it up as potential energy in the various complex compounds which they form in; while animals draw their supplies of force entirely from those compounds in which it has been stored up by plants, and from which it is set free again when they are decomposed in the organism.

In a word, the plant converts the actual energy of the sunlight into the potential energy of organic compounds, the animal converts the potential energy of the organic compounds into actual energy, which manifests itself as heat, motion, electricity, etc.; in the plant the spring is coiled up, in the animal it uncoils, exerting an amount of