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A spectrum (plural ''spectra'' or ''spectrums''〔(Dictionary.com ). ''The American Heritage Dictionary of the English Language'', Fourth Edition. Houghton Mifflin Company, 2004. (accessed: January 25, 2008).〕) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word was first used scientifically within the field of optics to describe the rainbow of colors in visible light when separated using a prism. As scientific understanding of light advanced, it came to apply to the entire electromagnetic spectrum. Spectrum has since been applied by analogy to topics outside of optics. Thus, one might talk about the ''spectrum of political opinion'', or the ''spectrum of activity'' of a drug, or the ''autism spectrum''. In these uses, values within a spectrum may not be associated with precisely quantifiable numbers or definitions. Such uses imply a broad range of conditions or behaviors grouped together and studied under a single title for ease of discussion. In most modern usages of ''spectrum'' there is a unifying theme between extremes at either end. Some older usages of the word did not have a unifying theme, but they led to modern ones through a sequence of events set out below. Modern usages in mathematics did evolve from a unifying theme, but this may be difficult to recognize. ==Etymology== In Latin ''spectrum'' means "image" or "apparition", including the meaning "spectre". Spectral evidence is testimony about what was done by spectres of persons not present physically, or hearsay evidence about what ghosts or apparitions of Satan said. It was used to convict a number of persons of witchcraft at Salem, Massachusetts in the late 17th century. The word "spectrum" () was strictly used to designate a ghostly optical afterimage by Goethe in his ''Theory of Colors'' and Schopenhauer in ''On Vision and Colors''. The prefix "spectro-" is used to form words relating to spectra. For example, a spectrometer is a device used to record spectra and spectroscopy is the use of a spectrometer for chemical analysis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「spectrum」の詳細全文を読む スポンサード リンク
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