Till-. OPTICAL NETWORK EXPLAINED 211 



area remains visible as a light circle. Now, if two or more 



O 



granules of ink an- lying close together, and the tube be 

 raised or lowered, the granules pass into a network of just 

 as many meshes as there were granules originally. The 

 explanation of this appearance must be sought in wbat 

 lias been pointed out above, that instead of the granules 

 appear clear diffraction circles, which partly overlap one 

 another and by their dark edges produce the appearance of 

 a meshwork. Now since in the examination of a layer of ink, 

 such as has been described above, there are numerous granules 

 not in sharp focus which must present the appearance of 

 such a network produced by the diffraction circles, I think 

 that the reticular appearance depends essentially on the 

 last-mentioned optical phenomenon. 



The formation of net-like structures just described, which 

 possess no reality, but depend only upon optical phenomena 

 connected with the peculiarities of microscopic vision, has 

 been followed out by me still further, since a knowledge 

 of them, which has hitherto been quite neglected, must be 

 exceedingly important for estimating the value of reticular 

 structures, and may indeed render it a matter of doubt 

 whether all reticular structures are not on the whole only 

 due to optical phenomena. It was for these reasons that I 

 discussed the question in detail above, and, as I believe, 

 settled definitely whether the appearance of reticular struc- 

 ture in the foams depended in reality upon their frothy 

 nature or not, since the possibility that dense granular 

 deposits might produce the appearance of a reticulum w;i- 

 not to be lost sight of. 



If one prepares an emulsion of oil-drops as minute as 

 possible by shaking up a little olive oil with 1 per cent 

 solution of caustic soda, and examines it in a thin layer with 

 the strongest magnifications, the following facts may be ob- 

 served. If the layer of fluid is thick, all the droplets are 

 found in violent molecular movement ; but if the glass be 

 pressed down more strongly by drawing off fluid, the larger 

 drops are held still, and round them collect groups of the 

 smallest drops, which are now in like manner quiescent. By 

 focussing accurately the median plane of these droplets it is 



