John sunlight tetrahedron

That gives us a general classification for any number, showing a broad environment in which we can do a specific interpretation of the unique higher number.

The great variety of organic compounds, and the enormous amount of investigation that has been done on them, can be appreciated in no better way than by a look at Beilstein's handbook of organic chemistry (see References). The fourth edition, which appeared in the 1920's, covers work up to 1910, and later work is quoted in supplements. Several suggestions are made in this article about compounds to look up, and the exercise is highly recommended. Proper use of Beilstein requires a knowledge of German, which was once a sufficient reason for all chemistry postgraduates to learn to read the language. Similar references exist in English, but they are not of the standard of Beilstein. Incidentally, a Handbuch in German was not one you could hold in your hand (all of Beilstein now seems to weigh about a ton), but one that is to be kept "at hand" for reference. Among the sciences, chemistry uniquely requires a reference like Beilstein.

Optical telescopes are either refractors or reflectors that use lenses or mirrors , respectively, for their main light-collecting elements (objectives). Refractors are effectively limited to apertures of about 100 cm (approximately 40 inches) or less because of problems inherent in the use of large glass lenses. These distort under their own weight and can be supported only around the perimeter; an appreciable amount of light is lost due to absorption in the glass. Large-aperture refractors are very long and require large and expensive domes. The largest modern telescopes are all reflectors, the very largest composed of many segmented components and having overall diameters of about 10 metres (33 feet). Reflectors are not subject to the chromatic problems of refractors, can be better supported mechanically, and can be housed in smaller domes because they are more compact than the long-tube refractors.

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