We said that the appearances in the starch-grains led Nägeli to suppose that their molecules are not spherical but polyhedral, and the question naturally arose whether they are really crystalline. The point could be settled by the use of polarised light, to which different observers had already turned their attention. Erlach in 1847, Ehrenberg in 1849, had employed polarised light for the determination of microscopic objects, without however arriving at any conclusions on the subject of molecular structure; Schacht indeed at a later time declared such observations to be a pretty amusement, but without scientific value. But soon we have once more one of von Mohl’s careful and solid investigations (‘Botanische Zeitung,’ 1858), in which with the aid of technical improvements in the apparatus he arrived at conclusions respecting the nature 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 observations on cell-formation; he was content once more to observe thoroughly and correctly, to describe what he observed carefully, and to connect it with proximate physical principles in such a manner as to supply rather a classification of phenomena, 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 representation 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 Nägeli, 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.