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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    combated by Aristotle as early as 350 B.C. He maintained, that light is not a material emission from any source, but a mere quality of, or action (evepyeia) of a medium which he called the pellucid Although the reasoning of Aristotle was very superficial, yet he is entitled to considerable credit for his sagacious, though vague specula- tions regarding the nature of light and various optical phenomena. He may to some extent be regarded as having in a haphazard manner anticipated the undulatory theory of light, which was established two thousand years afterwards by the labours of Huygens, Young, and Fresnel. 4. Knowledge of the Ancients.—The principal phenomena of the rainbow, halos, etc., had not escaped the notice of the ancients, who classed all these appearances under the common denomination of meteors. Aristotle^ attributed these phenomena to the reflection of the sun's rays from drops of rain, and observed that a rainbow may be made by the spray from an oar, and that in this case it will be visible ^ There is then, let us begin by saying, something which is pellucid. And by pellucid is meant something which is visible, not visible by itself (to speak without further qualification), but visible by reason of some foreign colour which affects its neutral pellucidity. Of this character are air and water, and also many among solid bodies, water and air being pellucid not in virtue of their qualities as water or air, but because they both contain the same element as constitutes the everlasting Empyrean essence. Light is then the action (ivepyeia) of this pellucid qua pel- lucid; and whenever this pellucidity is present only potentially, there darkness also is present. . . . Thus we have shown light to be neither fire, nor body gener- ally, nor even the effluvium or emanation from any body (since even in this case it would be a body of a kind), but only the presence of fire, or something like it, in that which is pellucid ; two bodies being unable to exist at one and the same time within the same space. . . . Darkness in fact is really the removal of such a posi- tive quality from what is pellucid, so that light must necessarily be its presence. Empedocles, therefore, and many others who have followed him, have not described the phenomenon correctly in speaking of light as moving itself, and as coming some time or other without our knowing it into existence between the earth and the surrounding air. . . . And the pellucid itself is also similarly dark, but it is so not when it is pellucid in actuality, but only so potentially ; for it is one and the same element which is at one time darkness and at another time light. . . . Colour therefore is not visible without the presence of light ; this indeed we saw was the essential character of colour that it is calculated to set the actually pellucid in movement; and the full play of this pellucid constitutes light. . . . Vision is the result of some impression made upon the faculty of sense ; an impression which cannot be effected by the colour itself as perceived, and must therefore be due to the medium which intervenes. An intervening substance then of one kind or another there must necessarily be ; and were this intervening space made empty, not only will the object not be seen exactly, but it will not be perceived at all {Aristotle's Psychology, book ii. chap, vii., by Edwin Wallace, M.A,, Cambridge University Press Series, 1882). 2 Meteor, lib. iii. cap. ii.