50 On the Direction of the Vibrations of the Luminiferous ^ther. 



exhibited be examined through the dichroscopic lens in all azi- 

 muths perpendicular to the axis. Of the two images of the 

 dichroscopic lens, the ordinary one being polarized in the direc- 

 tion of both images, and the extraordinaiy one perpendicular to 

 this direction, the images appear differently coloured by the two 

 different tints of the dichromatic crystal. 



The position of the eye and dichroscopic lens 

 remaining unchanged, if the ciystal be turned 

 around the axis AA' the phsenomena remain the 

 same : we observe the upper image in the ad- 

 jacent figure, polarized in the direction of the 

 axis and of the transparent colour A, and the 

 lower image E polarized perpendicular to the axis 

 and possessing the other transparent colour of 

 the crystal. 



When the crystal is observed in the direction of the axis, the 

 two images of the dichroscopic lens appear of the same colour, 

 that of A, or of the ordinary image in the observation perpen- 

 dicular to the axis. 



2. Proposition. — The vibrations of the cet her are perpendicular 

 to the plane of polarization. 



3. Proof. — In ordinary light the vibrations of the sether 

 occur in all azimuths perpendicular to the direction of propaga- 

 tion. In plane-polarized light the vibrations are all made in one 

 and the same plane. 



In every observation perpendicular to the axis two rays of 

 light appear, which are polarized at right angles to each other. 

 For everj^ single observation, therefore, we have two planes of 

 vibration. In the homogeneous crystal each of these is distin- 

 guished by a peculiar colour. For all observations round about 

 the axis we have one dii'ection of vibration coinciding with the 

 axis, and an infinite number of directions of vibration which lie 

 in the plane perpendicular to the axis. 



In the direction of the axis only one colour appears, but it is 

 observed in all azimuths. To it evidently belong all vibrations 

 which are perpendicular to the axis, and which exhibit the same 

 colour when the line of view is perpendicular to the axis. 



The other colour is not at all observed in the direction of the 

 axis, but in all azimuths round the latter. The only vibrations 

 which can give rise to it are those which take place in the direc- 

 tion of the axis. Observation shows that this colour is polarized 

 perpendicular to the axis. It is, however, caused by vibrations 

 in the direction of the axis, and consequently the vibrations stand 

 perpendicular to the plane of polarization, which was to be proved. 



The method of proof could also be conducted as follows : — 

 Let the three lines perpendicular to each other be AA' the axis, 

 BB' the longitudinal diagonal, and CC the transverse diagonal. 



