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The accumulation of facts in that department of science which immediately considers the chemical changes produced by the solar rays, has been so great, that the necessity of collecting and collating them, has been felt by all who were in the least degree interested in the progress of the inquiry. Whether I have executed this task satisfactorily or otherwise, is for the public to decide. I have endeavoured, as this is the first History of Photography which has been published, to give to every one his full share in these discoveries, which have accelerated the advancement of that art ; and although in many cases my interpretation of phenomena may differ from that given by the observers themselves; in all, I have been most careful to adhere to their own expression of the facts.

I have, throughout the principal portion of this Work, laboured under the difficulty of being obliged to speak of photographic phenomena as resulting from the agency of Light, being at the same time fully satisfied that they were to be referred to a principle which possessed none of the characters of Light or HEAT, but which was intimately mixed with these elements in the solar rays. This subject is fully discussed in the third division of the Volume; but


I refer to it now, for the purpose of explaining, that the implied contradiction is only a submission to the generally received idea, for the purpose of rendering the inquiry intelligible to every reader.

It is due from me, that I acknowledge the kind and generous assistance which, through every stage of these experimental inquiries, I have received from Sir J. F. W. Herschel. The readiness with which that philosopher has communicated his discoveries, and the free and candid manner in which he has favoured me with his views, claims this expression of my feelings, as the only way in which I can sufficiently show the value at which I estimate his liberal endeavours to assist a very humble experimentalist, in a path of inquiry in which, by his own laborious and ingenious researches, he has established his high pre eminence. To Professor Wheatstone I am also indebted for some valuable matter connected with the early history of Photography.


Falmouth, March 11. 1844.


Bacon declared, — there can be no real knowledge but that which is based on observed facts; and the correctness of this has been admitted by all eminent thinkers since his time.

A fact new to man's knowledge, the blackening of a white salt of silver-presents itself, and naturally the discoverer seeks to find the cause to which this phenomenon is due. The salt of silver remains perfectly white so long as it is kept in darkness; but it blackens when it is exposed to sunshine. Consequently the change of colour, which is all that was at first observed, appearing to be connected with Light, man devises an interpretation of the phenomenon-forms a hypothesis—and says, “the calx of silver separates the phlogiston from the light, and retains the superfluous phlogiston of light.” Men of science have changed their views; but their mode of reasoning on this phenomenon is as much guided by preconceptions as was that of Scheele when he was disposed to refer the decomposition of chloride of silver to phlogiston.

Conjecture is a process common to every mind; we all frame hypotheses as we endeavour to advance from effects to causes. The strictest inductive philosophy allows of this; but the hypothesis must not be permitted to take the place of a theory, which is an explanation based on a large number of well-observed facts. Newton's fundamental rule was, “ No more causes, nor any other causes of natural effects, ought to be admitted, but such as are both

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