Page:Proceedings of the Royal Society of London Vol 1.djvu/388

Rh In a third set of experiments, conducted in the same manner, the under surface brought into contact with the prisms, consisted of mica, rendered nearly cylindrical by being bent over a cylindrical surface.

From the irregularity in the form of the mica, that of the colours was also irregular; but they served to show the increase of extent to which such appearances may be rendered visible by corresponding change of the angle of the prism.

Dr. Herschel is consequently of opinion, that any one who could object to the admission of critical separation as the cause of the phenomena under consida'ation, cannot have paid sufﬁcient attention to the modifying power of the suhjacent reﬂecting surface, which is so essential to their formation.

If any one is disposed to assume that the rings must arise from some other cause than critical separation, unless it can be shown how rays critically separated can reach the eye, the author thinks it is not to be expected that he should trace them through a most intricate complication of reﬂections from curve to curve, when it has been shown, in the second part of this paper, that even with streaks produced by contact of two plain surfaces, it would be an endless attempt to follow them. He accordingly thinks it sufﬁcient to have proved, to his own satisfaction, two essential points; ﬁrst, that colours separated critically may be formed into tings, when modiﬁcation will increase the ﬁeld of visibility to any extent beyond the limits of critical separation.

Enough (says the author) has been said to prove that the phenomena of coloured rings, and other phenomena that have been ascribed to certain ﬁts of easy reﬂection and easy transmission, admit of the most satisfactory explanation, by substituting the solid principle of the critical separation of the different colours, in the room of these ﬁts.

In the ﬁrst communication which Mr. Knight made to the Society in the year 1795, he showed that the period to which the existence of any one variety of fruit could be prolonged by grafting, was limited; and that any portion detached from an old tree, and transplanted upon a young stock, was not thereby restored to what can, with propriety, be called a young tree.

Mr. Knight’s endeavours have, since that time, been directed toward ascertaining which of the several organs it is that ﬁrst fails in the performance of its proper ofﬁce in consequence of age, and the result of his experiments forms the subject of the present letter.

In the prosecution of these inquiries Mr. Knight bears constantly in mind the analogy that subsists, in many respects, between the organs of animals and those of vegetables; for though it may not be in his power to avail himself of any assistance to be derived from