Photography by Robert Hunt Francis Peabody

Author:Robert Hunt, Francis Peabody
Language: eng
Format: epub, pdf
Publisher: R. Griffin & Co.
Published: 1853-03-25T05:00:00+00:00

twenty-four hours by a clock movement. It has a triangular aperture cut down its whole length, as shown in the figure, and it carries the scale of the instrument, which is made to spring closely against the prepared paper. This scale or screen is composed of a sheet of metal foil between two sheets of varnished paper, and is divided into one hundred parts longitudinally, every other part being cut out, so as to admit the light to the prepared paper without any transparent medium intervening. The lengths of the extreme divisions, measuring round the cylinder, are proportioned to each other as one to one hundred; consequently the lower division will be one hundred times longer passing over its own length than the upper one over its own length, and the lines of prepared paper upon these divisions will, of course, be exposed to the light for times bearing the same proportion to each other.

Now, as the sensitiveness of the paper can readily be adjusted, so that the most intense light will only just tint it through the upper division during its passage under the opening, and the most feeble light will produce a similar tint through the lower division during its passage, the number of lines marked on the paper at any given time will furnish a comparative measure of the intensity of solar light at that time, and may be registered as so many degrees of the Heliograph, the name Mr. Jordan

has given his instrument, just as we now register the degrees of the thermometer.

An instrument of this kind was made by me for the British Association, and experiments carried on with it, at intervals, for some years. Many of the results were very curious, but the instrument being placed at the Observatory at Kew, the observations were unfortunately discontinued. It is believed that, with an instrument properly constructed, the details of the one employed were capable of much improvement; many very remarkable alterations in the relative chemical power "of the solar radiations would be detected. From the indications I have obtained, I believe there exists a constant law of change, and that the correct expression of the phenomena is given in the following passages—the concluding summary of my Eeport on this subject to the British Association at Edinburgh:—

" It will be evident that the question which assumes the most prominence in our consideration of these remarkable phenomena is that of the identity or otherwise of light and actinism.

" Eresnel has stated that the chemical effects produced by the influence of light are owing to a mechanical action exerted by the molecules of aether on the atoms of bodies, so as to cause them to assume new states of equilibrium dependent on the nature and on the velocity of the vibrations to which they are subjected.

" Arago says, it is by no means proved that the photogenic modifications of sensitive surfaces result from the action of solar light itself. These modifications are perhaps engendered by invisible radiations mixed with light properly so called, proceeding with it, and being similarly refracted.

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