676 Transactions of the Socieiij. 



pictures are both taken at precisely the same distance from object 

 to screen, and would be of precisely equal magnifying power, but 

 that the diffraction lines are best seen in a focal position of the 

 objective slightly different from that required to show the hexagons, 

 and this produces unavoidably a trifling variation, which, however, 

 I think cannot exceed one per cent, of the power, if indeed it is so 

 great. I have added to the pair a third photograph, exhibiting 

 the whole of the frustule used for this experiment, magnified 730 

 diameters by Powell and Lealand's water-immersion |. 



