58 THE POPULAR SCIENCE MONTHLY. 



is tinted with color, which is most brilliant when the plate has been 

 turned through half a right angle from a neutral position. If one of 

 the Nicols be turned, the selenite remaining still, the color will fade 

 and entirely vanish when the Nicol has turned through half a right 

 angle. After this position the complementary color will begin to 

 appear, and will be brightest when the Nicol has completed a right 

 ano-le. 



The oolors so produced depend upon the thickness of the plate ; 

 thus, if we take a plate of selenite merely split and not ground to a 

 uniform thickness, we shall have a variety of tints indicating the thick- 

 ness of each particular part ; or we may, by a careful arrangement of 

 suitable thicknesses, produce a colored pattern of delicacy and variety 

 dependent only upon the skill with which the pieces have been worked. 



A plate of the same crystal worked into a concave form is interest- 

 ing as showing not only that the colors are dependent upon the thick- 

 ness, but also that when, with an increasing or diminishing thickness 

 of crystal, they have run through their cycle, they begin again ; in 

 other words, that the phenomenon is periodic. The field is then 

 covered with a series of concentric rings, each of which is tinted with 

 colors in a regular order. 



In all these instances it is clear, from the experiments themselves, 

 as well as from other experiments which form no part of our present 

 subject, that the modifications which light undergoes are due to the 

 internal structure of the crystals used. And it becomes a question of 

 interest whether it be not possible, by some mechanical process, per- 

 formed upon a non-cystalline substance, such as glass, so far to imitate 

 a crystalline structure as to reproduce some of the optical results 

 already shown. For this purpose let us take a bar of glass. On inter- 

 posing it in its natural state between the Nicols when crossed, we find 

 that no effect is produced in the dark field upon the screen. If, how- 

 ever, I merely press it as though with the intention of bending or 

 breaking it, there will be at once brought about a condition of strain 

 capable of affecting the vibrations of the ray falling upon it, to such a 

 degree that some of them will find their way through the screen. And 

 this result may be explained on precisely the same mechanical princi- 

 ples as in the case of the crystal. The effect may be heightened by 

 placing the piece of glass in a vice, and screwing it up so as to bend 

 or compress it to a greater degree than w r as possible by the hand alone. 

 "When this is done the direction and even the relative amount of 

 torsion or compression of the different parts will be noted down as it 

 were by the forms and hues of the figures thrown upon the screen. 



The same kind of effect is shown by a piece of glass unevenly 

 heated; but better still by glass which has been rapidly and unevenly 

 cooled unannealed glass, as it is called. In the pieces now before 

 you, the outside, having become first cooled and solidified, has formed 

 a rigid framework, to which all the interior has been obliged to con- 



