Notes. 557 



This observer considered these movements to be due to an 

 exudation of mucilage, and the first two to the formation durin</ 

 the motion of a filament of mucilage, by which the desmid is tem- 

 porarily attached to the bottom, and which gradually lengthens. 



These four kinds of movements are very easily explained by 

 the theory of the evolution of gas ; and by regulating the con- 

 ditions they can be exactly reproduced in the artificial desmids 

 made of aluminium. In this case strips of thin aluminium foil 

 should be used. When the gas production is very strong at one 

 end, the desmid will be raised to a vertical position and will take 

 up oscillating or circular movenu nts. 



If we now pass to a consideration of like movements in the 

 Cyanophycese, the same explanation holds true for Oscillaria, 

 which often takes up a waving or circular motion when attached 

 at one end. This movement is well described by Griffith and 

 Henfrey * as follows : — " The ends of the filaments emerge from 

 their sheaths, the young extremities being apparently devoid of 

 their coat ; their ends wave backward and forward, somewhat as 

 the forepart of the bodies of certain caterpillars are waved when 

 they stand on their prolegs with the head reared up." The authors 

 attribute this motion to " irregular contraction of the different parts 

 of the protoplasm." 



The free- swimming species of Nostoc all have a spontaneous 

 power of active motion in water, and in all of the filiform orders 

 of the Cyanophycese detached portions of the filament known as 

 hormogones also have the power of spontaneous motion. All of 

 these movements are undoubtedly the effect of the evolution of 

 oxygen gas. 



On "An Optical Paradox." 



Dr. G. Johnstone Stoney, F.E.S., takes the subject f up from the 

 point of view of "Flat-wavelet Eesolution," and remarks that 

 Lord Eayleigh's experiment t may be conveniently adapted to the 

 Microscope. 



The image of L in the telescope T of the diagram, fig. 129, is formed 

 by the mutual interference of all the undulations of flat wavelets, 

 which enter the telescope in the direction of the so-called " rays " 

 that proceed from the image of A at C. The image of A at C is 

 the spurious disk and diffraction appendages formed by lens L. 

 The outline of L will be properly seen so long as the telescope 

 objective admits, in addition to the spurious disk, some part of the 



* Micrographic Dictionary, p. 501. 

 t Phil. Mag., July 1905, pp. 126-8. J See this Journal, ante, p. 417. 



Oct. 18th, 1905 2 



1' 



