A description of the dipleidoscope, or double-reflecting meridian and altitude instrument : with plain instructions for the method of using it in the correction of time-keepers / by Edward J. Dent.

  • Edward John Dent
Date:
1845
Catalogue details

Licence: Public Domain Mark

Credit: A description of the dipleidoscope, or double-reflecting meridian and altitude instrument : with plain instructions for the method of using it in the correction of time-keepers / by Edward J. Dent. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    METHOD OF PLACING THE DIPLEIDOSCOPE, AND DIRECTIONS FOR TESTING. To illustrate and explain the method of placing the Dipleidoscope in the meridian, let us suppose the place of residence to be Dover. The lon- gitude of the abode, in time, having been ascertained by reference to the ordnance-map, a good watch or chronometer, set to the time of the spot, is to be taken for the standard; and on the following day (or as soon after as the sun will permit), let us suppose the instrument is to be fixed. After it has been placed on a previously levelled surface, the following preparatory practice may he gone through, commencing about two hours before noon. Place the instrument on the unwrought end, with its two flat sides in the direction of the sun’s rays : a sheet of paper may now be held nearly opposite the front glass, at about two feet distance (care being taken not to obstruct the^rays of the sun from passing into the instrument), when the reflected image from the front glass wiU immediately be thrown upon the paper. The instrument should then be gently turned either to the right or left, until another similar image appear on the paper; these will mutually approach, until one is seen to cover the other, and they appear as one. If the observer then looks into the instrument (having the lowest side a (fig. 4) always towards him, and his eye protected by means of the darkened glass), he will perceive the sun reflected in it, as one luminous circular object. The principal advantage derived from previously throwing the images on the paper is, that it indicates to the observer the direction in which he is to, look into the instrument. By keeping his eye upon the sun, as exhibited in the glass, he will, after the interval of a few moments, perceive a second sun appear to pass away from the former:—these will presently be entirely detached, and two distinct suns will be seen. If he turn the instrument very gradually towards the west, the images may be so regulated as to re-approach, coincide, and separate again ; and thus aflbrd the opportunity of practising the complete observation described at pages 15 and 16. This experiment may be repeated at the pleasure of the observer, until he feels he has acquired the requisite skill for the permanent
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