The theory of light / by Thomas Preston.

  • Preston, Thomas.
Date:
1901
Catalogue details

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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    require that a return beam of light shall leave the second station and travel to the first. The most obvious way of securing this is to replace the second observer B by a reflecting surface, such as a polished mirror, so that when A uncovers his lantern the light issuing from it may fall upon the mirror and be reflected back at the second station without loss of time. Thus when A uncovers his own lantern he may observe its image by reflection in the mirror at the second station, and the interval between the uncovering of the light and the perception of the image is the quantity to be estimated. The whole problem is now reduced to devising an accurate method of estimating this interval. One obvious source of error is a personal one attending the observer, and arises in the uncertain estimate of the interval between the muscular effort in removing the screen and the instant at which he considers he has perceived the return flash of light. To eliminate this the screen used to uncover the light should in its motion be employed to cut off the return rays. For example, let the lantern be covered, and let the eye of the observer be placed close to it and directed so as to look into the distant mirror. Then if the screen be suddenly moved so as to uncover the lantern and cover the eye of the observer, the return flash will not be received by the eye if the time occupied in moving the screen from the lantern to the eye is less than the time required by light to travel to the distant mirror and return. The problem is now reduced to the devising of some mechani- cal method of moving a screen, or a train of screens, across the field of view so that the source of light and the eye of the observer shall be covered and uncovered in succession. This was first done in 1849 by Fizeau by using a toothed wheel, which rotated in the field of view in such a way that the opaque teeth passed before the eye and intercepted successively the light emerging from the source and that returning to the eye after reflection in the distant mirror. This method is explained in the following article. FizEAu's Method 299. General Ppineiples of the Method.—In the method adopted by M. Fizeau^ a beam of light from a source S (Fig. 216) is in- troduced through a collimator fixed to the side of a telescope, and after reflection at the surface of a plate of parallel glass inclined at 45° to the axis of the tube, the pencil comes to a focus at a point F, which is the principal focus of the object-glass of the telescope. It ^ H. Fizeau, Oomptes liendus, torn. xxix. p. 90, 1849.