164 Dr. H. Johnson on a Property in Plants 



defined dark object on a luminous ground, and the most perfect 

 and colourless vision of this object will be obtained in and near 

 the common axis of the eye and the lens. Now in this case 

 we have sensibly colourless vision, although the lens is not 

 achromatic, and although its chromatic aberration is increased 

 by whatever colour there may be in the eye itself. How much 

 more, then, should vision be sensibly colourless near the axis of 

 vision^ and with the eye alone, when we consider that it is 

 composed of substances which have a much lower dispersive 

 power than glass ! 



Mr. Powell has quoted the admirable paper of Dr. Maske- 

 lyne, in which, without referring to the physiological fact on 

 which I have proceeded, he regards the eye as a lens, and 

 calculates the amount of indistinctness in the image which it 

 forms. He has shown that the calculated dispersion, which 

 we believe to be even less than he makes it, is not incompa- 

 tible with distinct vision, and he has pointed out causes which 

 tend to diminish the injurious effects of this dispersion. But 

 though Mr. Powell quotes these results, he does not attempt 

 to call them in question, or to disprove them by other calcula- 

 tions founded on more recent measures of dispersive power; 

 and until this is done, great weight must be attached to the 

 reasoning of Dr. Maskelyne. 



After a careful perusal of Mr. Powell's Memoir, I have no 

 hesitation in stating that I continue to maintain the opinions 

 which, along with others, I have published on this subject; and 

 that I consider the non-achromatism of the eye as a fact as well 

 established as any other fact in natural philosophy. 

 Belleville, January 15th, 1835. 



I 



XXVI. On the General Existence of a newly observed and 

 peculiar Property in Plants^ and on its Analogy to the Irri- 

 tability of Animals, By Henry Johnson, M.D,* 



DO not know that it has ever been remarked, that, on di- 

 viding the stem of almost any herbaceous plant, a singular 

 separation of the divided segments uniformly occurs, and that 

 this separation continues until the stem withers and dies 

 from the loss of its moisture. 



It was in the autumn of 1827 that I first observed this 

 fact ; and from an opinion which at once occurred to me that 

 it was connected with the motive powers of the plant, I have 

 been induced, since that period, to pay much attention to the 



♦ Communicated by the Author. This paper is an abstract of a Memoir 

 read before the Ashmolean Society of Oxford. 



