398 Dynamic Theory. 



But when the ray passes into the crystal parallel with the optic axis, 

 there is no refraction, and then the retardation of one half the ray be- 

 ing greater than the other half, one follows the other in the same track. 

 This is one of the proofs that the vibrations of common light are trans- 

 verse and in all azimuths, and that the vibrations of these two retarded 

 rays are in different azimuths, and that the retardation arises from a greater 

 hampering of the vibration in one azimuth than another ; otherwise 

 there is no apparent reason why one half of the ray could have a greater 

 or less velocity than the other. But unless such difference of velocity 

 existed there could be no difference in the refraction of the two rays, and 

 no separation of them as there is when the light enters at an angle to 

 the optic axis. The peculiarities in the crystallization of the crystals 

 causing these results have in some cases been detected and pointed out. 

 Sir John Herschel pointed out the peculiar crystallographic structure in 

 the quartz that causes the right and left handed rotary polarization. l 

 Besides the crystals that possess one optic axis or direction of no re- 

 fraction, there are a great many that have two such axes. These are 

 called biaxal in distinction from the uniaxal. Saltpeter is one of the 

 biaxal crystals, the two axes having an inclination toward each other of 

 6. Of course the chromatic effects resulting from this peculiarity are 

 different from those of the other crystals. 



Fluorescence. It has been stated that there are tones of vibration of 

 the ether still higher in pitch than this 44th octave, which has been un- 

 der discussion. One of the proofs of this is, that when a ray of light is 

 analyzed by a prism, and a spectrum formed, photographs have been 

 taken by the invisible rays above the violet of the spectrum. These in- 

 visible rays being more refrangible than the colored rays, belong, of 

 course, to one or more octaves above the 44th. These rays were first 

 named the actinic rays, and it was originally supposed that they alone, 

 or chiefly, were concerned in the production of chemical changes, be- 

 cause if a strip of paper, saturated with a solution of nitrate of silver, 

 be exposed in the spectrum, the greatest change will take place in the 

 part exposed to the ultra violet rays, and the change will decrease 

 downwards in the spectrum and be least in the red end. It does not 

 follow from this, however, that the visible rays have no chemical power. 

 One set of rays affect one substance, and another set another substance. 

 The yellow rays are the most effectual in decomposing carbonic acid in 

 the interest of vegetation. 



Prof. Stokes, of Cambridge, England, discovered that certain sub- 

 stances exposed to the action of the invisible rays, emit light and be- 

 come visible. He called this property Fluoresence. A number of 

 bodies possess this property, among them sulphate of quinine, fluor 

 1 See Smithsonian Report 1862, p. 135. 



