"264 E. M. NELSON ON THE BLACK AND WHITE DOT. 



note that the black dot in the patch is at precisely the same 

 focal plane as the white dot on the rest of the valve. In general 

 a balsam mounted P. formosum has the white dot below, a 

 curious anomaly pointed out by Mr. Stokes, but the finding of 

 an example where the reverse w^as the case caused me, a few 

 days ago, no little astonishment. So far as I am able to measure 

 it accurately, the distance between the black and white dot is 

 pretty constant at 3A. It should be borne in mind that silex 

 has a smaller refractive index than balsam, so a sphere of silex 

 in balsam would act as a negative lens. 



Passing on to Mr. Rheinberg's carefully worked-out papers, we 

 are brought at once face to face with a single point which, 

 if it is a fact, seemed to me to offer a fatal objection to his 

 whole argument. If, for example, the white dot occupies the 

 precise position of the black dot, and vice versa, then there is 

 an end to the trellis-work theory. Now I have made, both long 

 ago, and also sirice reading this paper, most careful experiments, 

 and can come to no other conclusion than that these white and 

 black, or black and white dots, as the case may be, are exactly 

 superimposed on one another. Critical examination of the six 

 dots surrounding a single blocked-up dot on, say, a P. angidatum, 

 will probably convince any observer that they change their 

 appearances without alteriner their positions, thus proving the 

 superposition of the black and white dots. This is an experiment 

 easily performed, because the six dots round the black spot are 

 arranged in a hexagon, and whether a side or an angle of the 

 hexagon faces a particular direction of the field can be most 

 readily seen. For example, if with the white dot a side of the 

 hexagon faces the upper side of the field, but with the black 

 dot an angle, then the black dots have shifted over to the inter- 

 costals, but if, on the other hand, there is no change in the 

 orientation of the hexagon, when the dots are changed by focal 

 adjustment, then the black dots must be precisely superimposed 

 on the white. Under these circumstances it would seem that 

 Mr. Rheinberg's most ingenious and very carefully worked-out 

 theory cannot be regarded as an explanation of the phenomena 

 in question. 



Jov.rn. Quekett MlcroscoiUcal Club, Ser. \ Vol. VIIL, No. 50, A]^>vil 1902. 



'^J 



