50 COHESION. 



existing. You will be able to do it at once 

 but if you try to cut it across the crystals you 

 cannot ; by hammering, you may bruise and 

 break it up but you can only divide it into 

 these beautiful little rhomboids. 



Now I want you to understand a little more 

 how this is and for this purpose I am going 

 to use the electric light again. You see, we 

 cannot look into the middle of a body like this 

 piece of glass. We perceive the outside form, 

 and the inside form, and we look through it ; 

 but we cannot well find out how these forms 

 become so, and I want you, therefore, to take 

 a lesson in the way in . which we use a ray of 

 light for the purpose of .seeing what is in the 

 interior of bodies. Light is a thing which is, 

 so to say, attracted by every substance that 

 gravitates (and we do not know anything that 

 does not). All matter affects light more or 

 less by what we may consider as a kind of 

 attraction, and I have arranged "(fig. 18) a very 

 simple experiment upon the floor of the room 

 for the purpose of illustrating this. I have put 

 into that basin a few things which those who 

 are in the body of the theatre will not be able 

 to see, and I am going to make use of this 



