106 DISPLACEMENT INTERFEROMETRY BY 



until after the lapse of hours the particles have practically subsided, when the 

 retrograde motion seems to be equally prominent. Even when the liquid is 

 manually rotated clockwise with violence this motion ceases in a few minutes, 

 whereupon the counter-clockwise, red-blue motion, in the direction of the 

 impinging beam sets in vigorously. 



It suffices to add a few statistical remarks. The telescope may be adapted 

 for small distances by placing three diopter spectacle lenses in front of it. Its 

 external focal plane is then only about a foot off and within the liquid. The 

 ray seen in the ocular of the spectro-telescope may be regarded as coming 

 from a virtual slit within the cylinder; or else, on narrowing the incident 

 beam L to within a centimeter (in case of a cylinder 10 cm. in diameter), the 

 diffuse internal caustic has already been similarly narrowed down to a short 

 internal spectrum rb in the figure. Hence, if the solution rotates slowly about 

 the axis A, particles enter the red (r), and leave the blue (6) end, and are 

 therefore seen sharply in the spectrum traveling from red to violet. The 

 reverse is the case if G rotates in the clockwise sense. The small distance rb 

 is thus virtually magnified by the immense dispersion of the grating g (15,000 

 lines to inch) . Since the rays cross within the cylinder G, the motion from 

 red to blue will characterize all particles distinctly seen (focus) and rotating 

 counter-clockwise. Finally, this rotation corresponds in a general way with 

 the direction of advance of the light transmitted through the cylinder. 



To obtain some idea of the size of particles we may take the breadth of the 

 diffraction arrows in the ocular, which breadth will not usually exceed o.oi 

 cm. The particles may then be estimated as a fraction (say ^, 1^) of this. 



It would be simplest to refer the cause of persistence to a case of vortical 

 motion in the wake of the beam of light traversing the solution. But the 

 invariable occurrence, in the lapse of time, of motion in the middle layers of 

 the liquid in the direction of the impinging light, no matter how the liquid is 

 artificially rotated in the beginning, leaves this explanation unsatisfactory. 

 Such vortices would not be orderly and persistently equivalent to the effect of 

 a pressure in the direction of the beam of light. In case of a black body 

 and a solar constant of 3 gram-calories per minute, the energy per unit of 

 volume or the light pressure in question may be roughly estimated at 7 X io~ 5 

 dynes per square centimeter. Even if but a part of the energy is absorbed 

 by the liquid, this is by no means an insignificant pressure in a medium whose 

 internal friction vanishes with its motion. In fact, if the given estimate be 

 treated as a tangential force relative to the surrounding dark liquid, of about 

 o.oi viscosity, a speed of 7XIQ- 3 cm. /sec. (under normal conditions) would 

 correspond to the shear. One may therefore infer that speeds within a tenth 

 millimeter per second, about of the order observed, are not impossible. The 

 very slow but persistent regressive movement at the top and bottom of the 

 layer of liquid remains unexplained. 



Furthermore, I was unable to find any adequate correspondence between 

 the swiftness of the motion and the intensity of the impinging beam. Again, 

 the molecular radiometer, in which the thermal gradient is at the same time 



