THE ETHER AND MOVING MATTER 111 



the anticipated displacement calculated from the purely kinetical 

 principle of Doppler. The experiments of the former, as a first order 

 test, on the rotation of the plane of polarization of a ray after passing 

 through a pile of plates has perhaps offered the greatest difficulty to 

 the exponents of both theories in reconciling the observations with 

 the results which should follow from each theory. In this experi- 

 ment, performed in 1859, the optical systems was mounted so as to 

 be rotated about a vertical axis alternately from east to west, or 

 vice versa. This system consisted of the usual polarizing nicol or sen- 

 sitive tint-system and analyzing nicol between which were placed 

 several piles of plates and compensating systems for producing the 

 rotations and the magnifying of the same, and also for compensating 

 for the rotary dispersion and elliptic polarization of the transmitted 

 light which was polarized in an azimuth of 45. In a series of ob- 

 servations extending over some time the mean of the rotations of 

 the plane of polarization showed a maximum excess in the direction 

 toward the west at noon and at the time of the solstice. It is to be 

 noted that light from a heliostat was reflected into the system alter- 

 nately by two fixed mirrors when the system was rotated. This 

 required an interruption and readjustment of the heliostat during 

 a single observation, i. e. from east to west and west to east, the dif- 

 ference in the setting of the analyzer in the two positions to give the 

 same field of view being, of course, the effect sought for. Fizeau 

 refers to the irregularities arising from successive settings of the helio- 

 stat. The calculated effect was much below that which could have 

 been observed directly with the usual polarizing system. To magnify 

 any such effect, a second system of plates was used which gave an 

 amplification as high as eighty times. Thus any residual rotation from 

 whatever cause would receive the corresponding amplifications. Now, 

 in experiments with polarizing systems using sunlight as a source of 

 illumination, it has frequently been noted that any shift in the direc- 

 tion of the light through the apparatus, either due to a change in the 

 direction of the beam (arriving, say, from the heliostat) or to a shift 

 in the optical system itself, produced a change in the field of view, 

 whether with a half-shade system or otherwise. In the former the 

 match was destroyed, the change being of an order much greater 

 than that which Fizeau anticipated from calculation. Further, with 

 such limited beams of light, a mere shift of the eye may produce 

 an effect of similar magnitude. Hence, in all polariscopic experi- 

 ments where sunlight is used, it is absolutely essential that, during 

 any single observation, the ray of light pass through the system and 

 into the eye over exactly the same path. This Fizeau failed to carry 

 out, and this is entirely sufficient to explain the very great discrepan- 

 cies in his various series of observations, and probably the apparent 

 constant difference in the results of his settings in the two directions. 



