After those days of national exaltation, Pasteur returned to his crystals. He studied tartrates under the influence of certain ideas that he himself liked to expound. Objects considered merely from the point of view of form, may be divided into two great categories. First, those objects which, placed before a mirror, give an image which can be superposed to them: these have a symmetrical plan; secondly, those which have an image which cannot be superposed to them: they are dissymmetrical. A chair, for instance, is symmetrical, or a straight flight of steps. But a spiral staircase is not symmetrical, its own image cannot be laid over it. If it turns to the right, its image turns to the left. In the same way the right hand cannot be superposed to the left hand, a righthand glove does not fit a left hand, and a right hand seen in a mirror gives the image of a left hand. Pasteur noticed that the crystals of tartaric acid and the tartrates had little faces, which had escaped even the profound observation of Mitscherlich and La Provostaye. These faces, which only existed on one half of the edges or similar angles, constituted what is called a hemihedral form. When the crystal was placed before a glass the image that appeared could not be superposed to the crystal; the comparison of the two hands was applicable to it. Pasteur thought that this aspect of the crystal might be an index of what existed within the molecules, dissymmetry of form corresponding with molecular dissymmetry. Mitscherlich had not perceived that his tartrate presented these little faces, this dissymmetry, whilst his paratartrate was without them, was in fact not hemihedral. Therefore, reasoned Pasteur, the deviation to the right of the plane of polarization produced by tartrate and the optical neutrality of paratartrates would be explained by a structural law. The first part of these conclusions was confirmed; all the crystals of tartrate proved to be hemihedral. But when Pasteur came to examine the crystals of paratartrate, hoping to find none of them hemihedral, he experienced a keen disappointment. The paratartrate also was hemihedral, but the faces of some of the crystals were inclined to the right, and those of others to the left. It then occurred to Pasteur to take up these crystals one by one and sort them carefully, putting on one side those which turned to the left, and on the other those which turned to the right. He thought that by observing their respective solutions in the polarizing apparatus, the two contrary hemihedral forms would give two contrary deviations; and then, by mixing together an equal number of each kind, as no doubt Mitscherlich had done, the resulting solution would have no action upon light, the two equal and directly opposite deviations exactly neutralizing each other.