CHAP, iv.j 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. 



Nageli 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 

 A a 2 



