176 HALOS AND PARHELIA. [LeSSOn XXVIT. 



cylinders must constitute a very thin cloud uni- 

 formly extended : through which the sun, and 

 even the colour of the sky may be seen. 



The whole of Huygens's dissertation,from which 

 the above hypothesis is deduced, is much too long 

 to be given in this place. Those of my young 

 readers who wish to peruse it, are referred to 

 chap. xi. book I. Dr. Smith's Optics, 



LESSON 



