Physical Properties of Protoplasm 147 



toplasm that are frequently very different from the normal living 

 substance, has never been refuted. Hence, at least, it does not seem 

 that more than an approximation of the actual structure of pro- 

 toplasm can be attained by the use of fixatives. Besides, this method 

 is worthless as a means of investigating the physics of protoplasm. 



The papers that fall in the second group deal with the study of 

 living cells. Strassburger, 6 Wilson, 7 Foot and Strobell, 8 Lundegardh 9 

 and others have shown that many of the structural elements of the 

 mi to tic figure can be seen in living animal and plant cells. Numerous 

 investigators have pointed out the presence of granules, vacuoles, 

 and fibrils in various types of unfixed cells; but for the most part, 

 the studies on living cells have been made for the purpose of decreas- 

 ing the error due to the use of fixing reagents. 



All such investigations are open to several sources of error. 

 Hardy 10 writes that the process of dying produces structural changes 

 in the cell substance, since coagulation appears to occur in all dying 

 cells. Many cells are certainly quickly asphyxiated when mounted 

 for microscopical examination in the usual manner. I have been 

 able to overcome, largely, this source of error, by the use of an open- 

 end moist chamber, that does not appear to interfere with normal 

 respiration. 



A second source of error is due to the nature of the optical prin- 

 ciples involved in microscopical vision. Many years since Abbe u 

 demonstrated that the optical image is a diffraction pattern produced 

 by the object and that under certain conditions the image may be 

 quite different from the object. More recently, Porter, 12 experi- 

 menting under ordinary working conditions, has described a number 

 of interesting examples of this sort. Porter 12 says, "Images were 

 formed which were utterly false in their smaller details, and other 

 images were profoundly modified by the presence of structure lying 



6 STRASBURGER: Zellbildung und Zelltheilung, Jena, 1880, iii. Auflage. 



7 WILSON: Journal of morphology, 1899, xv, Suppl. 



8 FOOT and STROBELL: American journal of anatomy, iv, p. 199. 



9 LUNDEGARDH: Jahrbiicher fur wissenschaftliche Botanik, li, p. 236. 



10 HARDY: Journal of physiology, 1899, xxiv, p. 158. 



11 ABBE: Archiv fur mikroskopische Anatomic, 1874, ix, p. 413; Gesam- 

 melte Abhandlungen, 1904, i, p. 45. 



12 PORTER: Philosophical magazine, 1906, xi, p. 154. 



