POLARISED LIGHT THROUGH DOUBLE REFRACTING CRYSTALS. 183 



admitted to be among the most beautiful of the phenomena which the science 

 of optics can produce. 



When homogenous light is used it is well-known that the rings assume 

 entirely the colour of the light used, the spaces between the coloured rings 

 being black. 



The splendour of the phenomena, however, obtained by the use either of 

 ordinary or of homogenous light, is incomparably inferior to that displayed by 

 projecting the rings against the spectrum. The spectrum microscope is admir- 

 ably suited for this exhibition. 



The method I adopted was simply to place the section of the crystal imme- 

 diately over the eye lens of the microscope, and between it and the analysing 

 prism. 



The rings are thus seen of every colour in the spectrum, alternating with 

 jet black rings between each, those in the red being the broadest ; and the 

 breadth of the rings gradually diminishing to the most refrangible end of the 

 spectrum. 



It is impossible to give any satisfactory idea of the appearances by mere 

 description, and no little skill or labour would be required to paint any 

 adequate representation of the effects seen in some of the following combinations. 



Take, as an example, a section of a crystal of sugar which gives a very fine 

 system of rings. I have counted easily as many as forty-five when projected 

 against the spectrum. This crystal is one of those which gives in polarised 

 light two black brushes, not a black cross like Iceland spar. When the Nicol's 

 prisms are at right angles the brushes are at their maximum intensity, and the 

 spectrum with its series of rings is seen to be cut in two by the jet black 

 brushes. When the analyser is turned through 90° the brushes which would 

 now, if seen by ordinary polarised light, be white, are of every colour in the 

 spectrum acording to the part of it they fall upon, and shaded off at their sides 

 by a nebulous haze of colour through which the black rings are visible. 



In intermediate positions of the analyser the brushes become entirely nebu- 

 lous, so that the rings can be seen through their whole extent. In this position 

 of matters the circle appears divided into four quadrants, and the rings are 

 distinctly seen to be dislocated so to speak, i.e., the rings in the alternate 

 quadrants are pushed out so that each coloured ring in the one quadrant is con- 

 tinuous with a black ring in the next. 



This effect is still better seen by circularly polarising the light before its 

 passage through the crystal. The effect of this is a curious one. Instead of 

 the circle being divided into four alternate quadrants, it is now divided into 

 two semicircles, the rings in the one being alternate with those in the other. 

 The semicircles are separated by two narrow coloured brushes which revolve 

 with the analyser, and seem as if they swept out the black rings in the one 



