274 POLARISED LIGHT. 



most starch-grains grow more or less unevenly, one side growing 

 faster than the other ; consequently, the black cross is more or less 

 unsymmetrical, both in form and position. 



Colours of Starch-G-rains seen with Selenite.— When any of 

 the objects mentioned in the last section are viewed with selenite, 

 they appear coloured, but the colours do not form a cross. Instead, 

 the object appears to be divided into four quadrants, and one pair 

 of opposite quadrants is differently coloured to the other pair. 



Now, this appearance agrees exactly with what we should 

 expect from theory. In a round object of this class there will be 

 one diameter — O O' (PI. VII., Fig. 8) — along which the optic 

 axes of the object are parallel to the corresponding axes of the 

 selenite. Here the colours will be the same as we should get by 

 slightly increasing the thickness of selenite (just as in the case of 

 two selenites placed with their axes parallel). Along the perpen- 

 dicular diameter (£, E) the optic axes are perpendicular to the 

 corresponding axes of the selenite, and the colours are the same 

 as we should get by slightly decreasmg the thickness of the selenite. 

 Between these two diameters there must evidently be a pair of 

 diameters (//, U U)^ along which the colour of the object is the 

 same as that of the selenite, and these divide the object into four 

 quadrants, two of which exhibit the colours of rather thicker and 

 two those of rather tJminer selenite than that employed in its 

 examination. 



Conclusion. — The reader cannot fail to be struck with the 

 remarkable way in which the appearances seen with the micro- 

 polariscope fit in with the wave-theory of light. It has hardly been 

 possible in the present paper to give more than the barest outline 

 of the theory of polarised light, and I have purposely omitted 

 any reference to polarisation by reflection, rotatory polarisation, 

 many other phenomena which, though equally interesting to the 

 physicist, are not so familiar to the microscopist. For these I 

 must refer the reader to Mr. Spottiswoode's book and other more 

 technical works on the subject. 



The more deeply Polarised Light is studied, the closer becomes 

 the agreement between experiment and the wave-theory. For the 

 present, however, I can only conclude by expressing a hope that 



