260 FRAGMENTS OF SCIENCE. 



The liquid here employed is the iodide of ally], but I 

 might choose any one of a dozen substances here before 

 me to produce the effect. You have seen what may be 

 done with the nitrite of butyl. With nitrite of amy], bisul- 

 phide of carbon, benzol, benzoic ether, etc., the same blue 

 color may be produced. In all cases, where matter slowly 

 passes from the molecular to the massive state the transi- 

 tion is marked by the production of the blue. More than 

 this : you have seen me looking at the blue color (I hardly 

 like to call it a blue " cloud," its texture and properties are 

 so different from ordinary clouds) through this bit of spar. 

 This is a Nicol's prism, and it is to be wished that one of 

 them could be placed in the hands of each of you. Now, 

 this blue that I have been regarding turns out to be, if the 

 expression be allowed, a bit of more perfect sky than the 

 sky itself. On looking across the illuminating beam as we 

 look across the solar rays in the atmosphere, we obtain not 

 only partial polarization, but perfect polarization. In one 

 position of the Nicol the blue light passes freely to the eye ; 

 in the other it is absolutely cut off, the experimental tube 

 being reduced to optical emptiness. It is well to place a 

 black surface behind the experimental tube, so as to prevent 

 foreign light from troubling the eye. In one position of 

 the prism this black surface is seen without softening or 

 qualification ; for the particles within the tube are them- 

 selves invisible, and the light which they scatter is quenched. 

 If the light of the sky were polarized with the same per- 

 fection, on looking properly toward it through a Nicol we 

 should meet, not the mild radiance of the firmament, but 

 the unillumined blackness of space. 



The construction of a Nicol's prism is such that it 

 allows the passage of vibrations which are executed in a 

 certain determinate direction, and these only. All vibra- 

 tions executed at right angles to this direction are com- 

 pletely stopped : while components only of those executed 



