301 



The Black and White Dot Phenomenon. 

 By J. Rheinberg, F.R.M.S. 



(^Read March 2lst, 1902.) 



You will, I am sure, agree that, however trivial a matter may 

 appear to be on the face of it, if it has a direct bearing upon 

 more important general problems, it is worth thre:^h]ng out 

 and clearing up. 



Mr. Nelson in his excellent article, read at our meeting last 

 December {ante, p. 261), tells us that the black and white dot 

 phenomenon is such a matter, and this is my excuse for asking 

 your kind indulgence for another communication on the subject. 



I am, moreover, anxious to acknowledge certain valid criticisms 

 passed upon my paper of April last {ante, p. 113), and to modify 

 the views then put forward in some respects. 



In the first place, further experiments show that it is erroneous 

 to suppose that a cone of light impinging on a vertical wall of 

 silex forms a dark area on both sides of the partition. It appears 

 that, in conformity with the ordinary laws of geometrical optics,* 

 the dark area is only formed on one side, viz., that of the 

 denser medium, and that on the side of the rarer medium, on the 

 contrary, there is an equivalent space where the light is doubled 

 in brightness. 



Secondly, I have to admit an error of observation, since, as 

 Mr. Nelson pointed out, the black and white dot are coincident 

 in position. It is not quite such an easy matter to determine as 

 it would seem ; in fact, I find that a very able writer on 

 microscopic optics has also stated that those images shift ; yet I 



* An error has crept in on p. 113 (7 lines from bottom of the page) in 

 describing the diagrams on p. 114. This ought to be read as follows. " The 

 cases represented are when light impinges on a vertical j^artition of a 

 diatom — 



Figs. 1 and 4 at an angle greater than the critical angle, 



Figs. 2 and 5 at the critical angle, 



Figs. 3 and G at an angle less than the critical angle.' 



Journal Q. M. C, Series II.— No. 51. 20 



