368 



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 phe- 

 nomena under consideration, cannot have paid sufficient attention to 

 the modifying power of the subjacent 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 intri- 

 cate complication of reflections from curve to curve, when it has been 

 shown, in the second part of this paper, that even with streaks pro- 

 duced 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 sepa- 

 rated critically may be formed into rings, when modification will in- 

 crease the field of visibility to any extent beyond the limits of cri- 

 tical separation. 



Enough (says the author) has been said to prove that the pheno- 

 mena 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. 



On the Parts of Trees primarily impaired by Age. In a Letter from 

 Thomas Andrew Knight, Esq. F.R.S. to the Right Hon. Sir Joseph 

 Banks, Bart. K.B. P.R.S. Read March 22, 1810. [Phil. Trans. 

 1810,^. 178.] 



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 li- 

 mited ; and that any portion detached from an old tree, and trans- 

 planted 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 to- 

 ward 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 



