35 



liVater is best where practicable. It sbould be contained in a 

 deep vessel, so that most of tbe water is in a deep shade. The 

 colours can be seen very well in this way. In many cases it is 

 preferable to use dark glasses of a neutral tint, as they can be 

 used anywhere. The best way to use them is to stand in the 

 shadow of some building so as just to hide the sun. Then 

 with the aid of dark glasses the brilliancy of the sky is suffi- 

 ciently reduced to enable the observer to distinguish the 

 colours clearly. A very valuable method consists in the use of 

 nicol-prisms and selenite film, thus obtaining any desired tint 

 &s background, and allowing one tint at a time to appear from 

 the clouds. 



