28 PROTOPLASM 



accurately in the middle, they appear moderately light; 

 with a slightly deeper focus, lighter ; but with a higher 

 focus, as darker round points. But since even in a 

 strongly compressed froth a number of layers of alveoli lie 

 one over the other, with even the sharpest focus, alveoli 

 must come into vision at the three foci named. Now if 

 there lies, as will often occur, an alveolus under a sharply- 

 focussed nodal point — one, that is to say, seen with a high 

 focus — it will produce the effect of making the nodal point 

 appear as a rounded dark spot, which with the. slightest 

 lowering of the tube changes into a clear alveolus. 



The appearance of the nodal points depends in part at 

 least on this circumstance, yet without doubt another 

 optical phenomenon must come into consideration. For if 

 a layer of froth be studied, one so thin that it is formed 

 only of a single layer of the minutest alveoli, the nodal 

 points are distinctly noticeable, provided that the alveoli 

 of the froth are not altogether too fine (see Photogr. 

 I.) It follows from this that the nodal points are shown 

 up even without a layer of other alveoli under them. If 

 the middle of the alveoli be sharply focussed, the nodal 

 points appear just as full as the edges of the alveoH, no 

 darker than the latter, but distinctly thickened, correspond- 

 ing to the explanation above of the way in which three 

 fluid lamellae meet one another. If now the tube be 

 lowered in the slightest degree, the nodal points show up 

 as very dark points, resembling granules (see the Photogr.), 

 while the rims of the alveoli connecting them appear con- 

 siderably lighter, and the contents of the alveoli much 

 lighter still, of course. The appearance of dark granules 

 deposited in the nodal points is so strong that only the 

 complete absence of such granules in the finely drawn out 

 and apparently quite homogeneous marginal portion of such 

 a very thin layer of froth, or in other cases, the want of any 

 such granules in the parts of the oil that are not frothy, 

 puts an end to the idea of very minute granular depositions. 

 That we are here dealing with a special optical phenomenon, 

 and not with granular contents, or with a special structural 

 relation of the froths, is plain at once from the fact that 



