The theory of light / by Thomas Preston.

  • Preston, Thomas.
Date:
1901
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Licence: In copyright

Credit: The theory of light / by Thomas Preston. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    and to have been exceedingly slow in advancing the construction of optical instruments, almost all the refinements of the subject having been originated and developed within the past three centuries. Yet it cannot be doubted that some of the more striking of the fundamental phenomena were observed and studied in the earliest times of civilisa- tion. If the physical theories of light and vision were subjects too profound for their investigation, they could not fail to acquire some knowledge of the laws of the reflection of light and the formation of images. The attention of the most careless observer must have been attracted with wonder to the image of himself depicted in still water : and the reflected landscape, or the image of a few bushes on the margin of a lake might have afforded to the humblest inquirer an assemblage of observations from which the general laws of reflection could be easily inferred. Metallic mirrors, and even glass, seem to have been manufactured long before any of the speculations of the ancient philosophers were recorded. They are distinctly mentioned in the Old Testament (Exodus and Job). The invention of burning glasses seems to have speedily followed the art of glass-making. Aristophanes ^ m^entions them as early as 424 B.C. 3. Pythagopas, Empedoeles, Plato, and Aristotle.—The sources from which light is most copiously derived are the sun, stars, and terres- trial bodies undergoing combustion or heated to incandescence. Such bodies we say are self-lmninous or simply luminous, while non-luminous bodies are those which are not visible of themselves, but only when illuminated, that is, when in the presence of a luminous body. The former class we say emit light, while the latter do not. We thus distinguish between luminous and illuminated bodies. Simple as it may appear to us to regard a luminous body as the source of some influence, which, acting on the eye, excites the sense of sight, much doubt appears to have existed among those who first investigated the subject as to whether objects become visible by means of something emitted by them, or by means of something issuing from the eye of the spectator. According to the opinion of Pythagoras ^ Comedy of the Clouds, Act II. (performed 424 B.C.) : Strepsiades. You have seen at the druggist's that fine transparent stone with which fires are kindled ? Socrates. You mean glass, do you not ? Strep. Just so. Soc. Well, what will you do with that ? Strep. When a summons is sent to me I will take this stone, and, placing myself in the sun, I will, though at a distance, melt all the writing of the summons. (The writing was then traced on wax spread over a solid substance.) Pliny also mentions globes of glass, which, when held to the sun, produced com- bustion, and Lactantius (303 a.d. ) states that a glass globe, filled with water and held to the sun, could light a fire even in the coldest weather.