368 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



where the light has been unable to get through from the side of greater 

 density ; and darkness is also brought about on the denser side of the 

 partition because throughout the whole of a certain solid space a direct 

 and a reflected wave-stream, emanating from the same points of the light- 

 source, meet in opposite phase. Figs. 83-85 represent a diatom denser 

 than the imbedding medium ; in figs. 86-88 the imbedding material has 

 the higher refractive index. In figs. 83 and 86 the light impinges at an 

 angle greater than the critical angle ; in figs. 84 and 87 at the critical 

 angle ; in figs. 85 and 88 at an angle less than the critical angle. The 

 closely shaded portions (when uncrossed by other lines) show where no 

 light has been able to get through ; and it will be seen that no light 

 reaches a space in the upper surface plane of the diatom just at or near 

 one of the edges of the vertical walls forming the partition. 



Fig. 83. 



Fig. 84. 



Fig. 85. 



Fig. 86. 



Fig. 87. 



Fig. 88. 



It will further be observed in figs. 83 and 84 and also in figs. 86 and 

 87, that the rays which impinge on to one of the vertical partitions are 

 thrown back on themselves. A change of phase occurs where they are 

 then reflected, and, if we take any point where one of these rays meets 

 one of the unreflected rays, we find the two have travelled the same 

 length of path and, being now in opposite phase, they cancel one another 

 and produce darkness. Thus darkness is formed in the surface plane of 

 the diatom both sides of the partition, though due to different causes in 

 the two cases. There are, in fact, two bands of darkness which issue 

 upwards from an angle to each other from the horizontal plane in which 

 the top of the vertical partition lies, in the case of a diatom which is 

 studded with perforations — in other words, with vertical positions — there 

 is an immense number of the bands parallel to each other in each of the 

 two directions forming a sort of trellis-work of light and darkness. 

 And, as sections of trellis-work taken one below the other would show 



