in which many hues were simultaneously visible, 

 our diatom presents its spectrum tints successively 

 and in monochrome. 



In truth these are pure spectrum shades; which 

 is to say, they are due to diffraction; the frustules, 

 having the same power as has a prism or the grat- 

 ing of the spectroscope, split up the rays of re- 

 ceived "white" light and pass onward the beams 

 in their primary colors. The peculiarity in the 

 structure of P. angulatum that causes this phe- 

 nomenon is one which the present magnification 

 cannot reveal; so let us inquire with a lens of 

 higher power. 



Dispensing now with a dark background and 

 viewing the diatom by transmitted light, that is, 

 with the light passing straight through the object, 

 we behold in the place of a number of smaller 

 individuals on the slide, but a single specimen 

 extending clear across the illuminated circular 

 field. So close have we come to our subject at five 

 hundred diameters enlargement that there is room 

 for only one within the range of the constant circle 

 of light. But even at this magnification a sharp eye 

 is necessary immediately to detect any change. It 

 should become apparent, nevertheless, that the 



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