1886-87.] as showing Peculiarities of certain Diatoms. 59 



peculiar feature of the Abbe condenser is its non-achromatism,- 

 on account of which there is dispersion of the emergent rays 

 into coloured pencils, and through the different refrangibility of 

 the chromatic rays the focus is lengthened out along the axial 

 Hue, and consists of a series of points forming a complete 

 spectrum. Thus the red rays, being least refrangible, come to 

 a focus at a point on the axial line farthest from the condenser, 

 and would illuminate an object placed at that point with a 

 monochromatic red light. Similarly the blue- rays come to a 

 focus nearest to the condenser, and would affect an object in 

 their focus. In practice these changes are effected by moving 

 the condenser up or down. 



The appearance of diatoms upon dark-ground illumination 

 is very striking ; for, not only do we see them projected upon a 

 dark ground in the same way that we can see foraminifera, but 

 for the most part they present themselves either in monochro- 

 matic light or in variegated colours — that is, some stand out in 

 one colour only, others are lit up by several spectral colours. 

 Now, as the light which falls upon the diatom by transmission 

 through the condenser consists of decomposed light by virtue 

 of the aberrations of the combination, it follows that, upon 

 whatever horizontal plane of the spectral focus the diatom is 

 placed, a monochromatic light will fall. Thus, as we have al- 

 ready seen, if the diatom is placed at the focus farthest from 

 the light, it will be illuminated by the least refrangible ray 

 — namely red. Placed at the focus nearest to the light, the 

 illumination will be blue — the other end of the spectrum. Or, 

 practically speaking, by racking up the condenser you get blue 

 light, by racking down you get red. This is the actual effect 

 presented to the eye in the case of a great many diatoms, but 

 with others it is not so ; consequently it must be concluded 

 that, because the reflected light does not conform to the colour 

 of the incident ray, and that it is impossible to disperse or alter 

 the colour of a homogeneous ray by interference, there must be 

 something in the composition or form of the frustule itself to 

 produce this effect. 



In the first place, we know that diatoms are siliceous, and 

 this accounts for their iridescent and reflective power. For- 

 aminifera, on the other hand, are also iridescent, but because 



