774 SUMMARY OF CURRENT RESEARCHES RELATING TO 



but of the same colour as the light illuminating the object, which in this- 

 case is visually a bright red. If the previous diatom diffraction image 

 be now examiued under these new conditions it will be found that it is 

 just as blue with this visually red light as it was with white light, owing 

 to the fact that eosin transmits the blue rays freely ; but the midrib of 

 the diatom, and particularly any granular incrustation, such as may 

 usually be found at places between the valve and cover-glass, will be 

 tinted red, indicating that refracted rays or complete diffracted beams 

 enter into their image formation. 



The author considers that three important questions have now been 

 settled : Firstly, that the diffracted beams from certain structures are 

 free from refracted rays ; secondly, that sharp, distinct images may 

 result from such diffracted beams exclusively ; thirdly, that, where such 

 beams are sufficiently separated from the dioptric beam to permit of 

 their being eclipsed by a suitable screen, no image of the elements pro- 

 ducing them will be visible. 



The author discusses several experiments described in Wright's 

 " Principles of Microscopy, 1 '' and points out their "fallacies," but for 

 details of these criticisms reference must be made to his original paper. 



The author's conclusions are adverse to the dioptric theory, and he 

 reminds us that Abbe's knowledge of optics was so profound that he was 

 liable to assume an equal knowledge on the part of his readers. If 

 Abbe's later application of the diffraction theory to the images of coarser 

 details, earlier termed " absorption images," could be interpreted as a 

 denial that any refracted rays, outside the dioptric beam, entered into 

 the image at all, then, indeed, some modification of what is generally 

 understood as the Abbe theory is necessary, as most objects unquestion- 

 ably refract light outside the dioptric beam ; and not only do the 

 simplest laws of refraction require that these rays find a place in the 

 image, but there is no other way of accounting for what becomes of 

 them. It is more likely, however, that Abbe regarded this fact as self- 

 evident. At any rate, the important work he undertook was not to 

 refute the dioptric theory, but to supplement it by accounting for 

 phenomena which it then, as now, failed to explain. 



Brownian Movement and Molecular Constants. — J. Perrin and 

 Dabrowski * refer to the fact that molecular hypotheses anticipate that 

 identical granules agitated by Brownian movement will behave as a 

 function of the depth (as a perfect gas does under the action of gravity), 

 and that they will satisfy the equation 



log U ° = p N v (A - 8) g h ■ 



/l ill T 



where n Y and n are the concentrations of the granules in two layers at a 

 distance h apart ; v, the volume of the grain ; A - 8, its apparent 

 density (excess of true density over density of the fluid) ; ~N, Avogadro's 

 constant (number of molecules contained in any molecule-granule). 

 Experiments as to granules of gamboge have verified the above equa- 

 tiom and give for N the value 70' 5 x lO' 22 . At the same time, 



* Coniptes Rendus, cxlix. (1909) pp. 477-9. 



