CHAP, iv.] and of growth by intussusception. 355

and substance of cell-membranes, starch-grains, &c., which proved that in the hands of a reflecting observer perfectly familiar with the physics of polarised light the instrument is no toy, but a means for penetrating deeply into nature's secrets. Yet on this occasion also appeared that peculiarity in von Mohl which twenty years before had prevented him from founding a conclusive theory upon his profound and extended observa- tions on cell-formation ; he was content once more to observe thoroughly and correctly, to describe what he observed care- fully, and to connect it with proximate physical principles in such a manner as to supply rather a classification of pheno- mena, than a new and deeper insight into the essence of the matter. He wanted the creative thought, the intense mental effort, to arrive by analysis at the ultimate elements in the results of his investigations and to frame for himself a clear represen- tation of the inner structure of the organised parts. Von Mohl in this case also stopped short at induction and did not pass on to deductive and constructive elaboration of the question before him ; this was left to Nageli, as we shall see.

Meanwhile a more exhaustive work appeared in 1861 from the pen of Valentin on the investigation of vegetable and animal tissue in polarised light, in which the author, equipped with great knowledge of the subject itself and its literature, examined in detail the phenomena of polarisation, gave a good account of the instrument and the mode of using it, and explained generally the theory and practice of investigations of the kind. But he overlooked one fact noticed by von Mohl, that vegetable cell-membranes, through which rays of polarised light pass perpendicularly to their surface, show interference- colours, and this was sure to lead him to an incorrect explanation of their inner structure.

N�geli from 1859 onwards made the phenomena of polarisa- tion the subject of protracted study, practical and theoretical ; the results were published in 1863 in his ' Beitrage,' Heft 3, but he had in the previous year made known that portion

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