48 The Microscope. 



intermediate spaces are light brown, and tlie back- 

 ground is white. The prism continuing to revolve 

 till half way round^ exhibits the brown spaces turning, 

 and gradually forming into vivid black V's as at first ; 

 and as it revolves through quarters three and four, 

 appearances exactly similar to those described in 

 quarters one and two are exhibited. 



I now remove the slide of eel's scales for a few 

 minutes, and place that containing the selenite on the 

 stage, while the upper prism remains in the position 

 number one. The whole field of view, instead of 

 being simply colourless, appears of a rich yellow. I 

 begin to turn the prism the yellow fades into white, 

 then this changes to pearl-colour, and then to brilliant 

 blue, by which time the prism has only revolved an 

 eighth. Continuing to turn the prism, I observe the 

 blue becoming pale much more slowly than was the 

 case with the yellow, and the prism has reached nearly 

 three-eighths of the way round before the field of 

 view has changed to white. Then it quickly changes 

 to yellow, assuming the brightest shade of that colour 

 when the prism is just half way round. Then blue 

 comes with speed as before, and slowly fades, till 

 yellow takes its place in the original position. 



Now, leaving the selenite on the stage, I place the 

 eel's scale over it. What a richly-coloured object it 

 has become, all pink, orange, and green. Some traces 

 remain of the pair of Vs, which would appear there 

 if the selenite were absent, but instead of being jet 

 black they are orange, the inside of the Y ; s is pink, 

 and the broad outer spaces are emerald green, while 



