252 THEORY. OF MICROSCOPIC OBSERVATION. 



Y. 



THE PHENOMENA OF MOTION. 



THE observation of the Phenomena of Motion under the Micro- 

 scope has led to many false views as to the nature of these 

 movements. If, for instance, Swarm-spores are seen to traverse 

 the field of view in one second, it might be thought that they 

 race through the water at the speed of an arrow, whereas they in 

 reality traverse in that time only a third part of a millimetre,, 

 which is somewhat more than a metre in an hour. It must not,, 

 therefore, be forgotten that the rapidity of motion of microscopic 

 objects is only an apparent one, and that its accurate estimation 

 is only possible by taking as our standard the actual ratio between 

 time and space. If we wish, for the sake of exact comparison, to 

 estimate the magnitude of the moving bodies, we may always do 

 so ; the ascertainment of the real rapidity remains, however, with 

 each successive motion the principal matter. 



If a screw-shaped spiral object, of slight thickness, revolves on 

 its axis in the focal plane, at the same time moving forward, it 

 presents the deceptive appearance of a serpentine motion. Thus. 

 it is that the horizontal projections of an object of this kind, 

 corresponding to the successive moments of time, appear exactly 

 as if the movement were a true serpentine one. As an example 

 of an appearance of this nature we may mention the alleged 

 serpentine motion of Spirillum and Vibrio. 



Similar illusions are also produced by Swarm-spores and Sperma- 

 tozoa ; they appear to describe serpentine lines, while in reality 

 they move in a spiral. It was formerly thought that a number 

 of different appearances of motion must be distinguished, whereas 

 modern observers have recognized most of them as consisting of 

 a forward movement combined with rotation, where the revolution 

 takes place sometimes round a central, and sometimes round an 

 excentric, axis. 1 To this category belong, for instance, the sup- 

 posed oscillations of the Oscillariece, whose changes of level, when 

 thus in motion, were formerly unnoticed. 



In addition to these characteristics of a spiral motion it must, 



. Cf. on this point Nsegeli : " Beitrage," ii. p. 88. 



