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CONCLUSION.

Having now completed my description of all the published photogra-phic processes of any real value, and my own observations on each ofthem, nothing more remains to be done, but to draw attention to theimportance of the art, both practically and philosophically considered.A great number of important considerations press themselves upon me,but I shall only remark on those which have the most popular ten-dency.

We have seen that the beams which proceed from the sun, in theform of white light, are each one of them a combination of distinctrays, not only differing in colour, but having very opposite powers. For-merly these rays were divided into three classes, and called the “ calo-rific,” the “ luminous,” and the “ chemical,” under an impression thatthese particular functions were confined to the particular parts thusnamed. Recent observations have shown that the calorific action is notconfined to the red or “ calorific ” rays, but that it extends over thegreater part of the spectrum. The “ luminous,” or yellow rays, havelong been considered as being destitute of any chemical action ; but SirJohn Herschel , in a communication he has been kind enough to maketo mo, and which I have his permission to publish, says, “ I have ob-tained photographic actions on certain papers, not argentine, which arelimited not to the ‘ chemical,’ not to the ‘ calorific,’ but to the ‘ lu-minous ’ rays, i. e. which seem to be produced exclusively, or nearly so,by those rays which affect the organ of sight. These papers are pre-pared with substances of vegetable origin ; and though at present I donot see how this can become serviceable in the arts, it strikes me asscientifically of considerable importance.” This fact is to mo singularlyinteresting, the more so, that in my experiments on the effects of lighton vegetation, I liavo detected powers in the “ luminous” rays which arehighly destructive to the germination and the growth of plants. Thisinquiry I am now pursuing, and I liopo the coming summer will enablome to add something to the many very remarkable discoveries recentlymade by Sir John Herschel and others.

Tho chemical action of light, so far fromjbeing confined to the mostrefrangible rays, we now know extends over tho whole spectrum, visibleand invisible, the action only being shifted from one ray to another,according to the substance upon which its peculiar functions are ex-erted. It is extremely difficult to explain many of the phenomena oflight by either of the rival theories; and as we proceed in our inquiries,the question of tho materiality or immateriality of light becomes moreand more complicated. A matter of much interest arises out of theseconsiderations, which is, are the different rays in similar electricalstates, or do they vary in this respect with their refrangibility ? Thosephilosophers who have adopted the undulatory theory of light, put the