80 dove's experiments OS THE 



the ring m the screw was withdrawn a turn, in order that the motion of 

 the rings, either away from the central point or towards it, might be the 

 more easily observed. 



The lamp having been lighted, the black cross began directly to open 

 in the centre ; the circular arcs in the second and fourth quadrant re- 

 ceded from the central point, whilst the fii'st and third approached it. 

 After some time the dark, arcs of the odd quadrants exactly corresponded 

 with the bright vacant spaces of the even ones; the light was circularly 

 polai'ized,and the difference of path was a quarter-undulation. Whilst this 

 was going on, with the exception of the points proceeding from the centre 

 which remained black, the dark cross had become brighter and brighter. 

 When it had entirely disappeared, the arcs, growing shorter at their 

 ends, had gradually advanced, so that the two black spots proceeding 

 from the centre formed with the parts approaching each other from the 

 two other quadrants, the inner ring, separated by four bright interven- 

 ino' vacant spaces. All the other rings were in the same state. The figure 

 o-iven bv the Iceland spar had thus changed, precisely as if the polarizing 

 prism had been revolved 90°; the light was therefore polarized linearly 

 and perpendicularly to tlie plane of primitive polarization : the difference 

 of path of both rays was a half-undulation. On a further heating, as the 

 difference of path became three quarters of an undulation, the light was 

 a^ain circularly polarized, with the difference, however, that now the rings 

 in the first and third quadrant were the nearest, those in the second and 

 fourth the more distant ; in which case the direction of the motion of the 

 arcs in the single quadrants naturally remained the same. Finally, when 

 the difference of path amounted to an entire undulation, the white cross 

 became darkened into a perfect black; the arcs previously separated 

 closed in whole circles; the light was polarized rectilinearly in the same 

 direction as at the beginning of the experiment. The lamp was now 

 removed and the opposite phsenomena were observed in regular succes- 

 sion during the cooling of the apparatus*; consequently the action of the 

 glass, becoming gradually heated from below upwards, upon the incident 

 lio-ht, is as follow s. The particles of aether, which at first vibrate recti- 

 linearly, begin to open into ellipses, the excentricity of which diminishes 

 continually, until they become circles. The axis which at first was the 



* Precisely the same succession of phaenomena may naturally be produced 

 by the gradual increase of pressure or its relaxation. With the plates, however, 

 which I had employed 1 was able to carry it only as far as a difference of path 

 of three quarters of an undulation in the proximity of the points of action of the 

 screw. On appl'ing a stronger pressure the plates broke. Now it is evident 

 that when a cooled glass plate, which in white light exhibits a regular series of 

 colours proceeding from black, is interposed, in homogeneous light the same 

 phsenomena will be observed in the plate of Iceland spar, if it be slowl^y moved 

 along before the aperture of the polarizing prism. The thicker the plate the 

 nearer to each other are the differently-acting vacant spaces. 



