1871.] of Aberration of Light through a Refracting Medium. 37 



micrometer is constructed on a plan arranged by myself, by which the 

 double observation in reversed positions of the instrument can be made 

 ■with great ease. The reference to the vertical is given by two spirit-levels, 

 both to be read at every single observation. The work of construction was 

 intrusted to Mr. James Simms, who carried it out with great ability. 

 Distilled water was supplied by H. W. Chisholm, Esq., Warden of 

 Standards. 



Had the result of the observations been confined to the determination of 

 an astronomical constant, or the variation of its value for different tele- 

 scopes, I should not have thought it worthy of communication to the Royal 

 Society. But it is really a result of great physical importance, not only 

 affecting the computation of the velocity of light, but also influencing the 

 whole treatment of the Undulatory Theory of Light. In this view I have 

 thought that an informal statement of the conclusions may be acceptable to 

 the Society, reserving for publication in one of the annual Greenwich 

 Volumes the details of the observations. 



The instrument was mounted in a small Occasional Observatory first con- 

 structed for the transit-instrument of Mr. Struve when he was engaged in 

 determining the longitude of Altona, and now planted on the " South 

 Ground " of the Observatory. The seasons at which the meridional zenith- 

 distance of y Draconis is most affected by aberration in opposite directions 

 are the Equinoxes. 



For understanding the following Table, it is to be remarked that an ap- 

 parent value of the Geographical Latitude of the Instrument is formed from 

 every observation, by subtracting the Observed Instrumental Zenith-distance 

 North of the Star from the Tabular Declination of the Star given in the 

 'Nautical Almanac.' The observed zenith-distance is affected with the True 

 Aberration as seen in the instrument, the tabular declination is affected 

 with the Received Aberration used in the computation of the ' Nautical Al- 

 manac,' and the apparent value of the geographical latitude is therefore 

 affected by the difference between the True Aberration as seen in the instru- 

 ment and the Received Aberration. If, therefore, under all circumstances, 

 and especially in the comparison of days when the sign of aberration has 

 changed, the apparent value of the geographical latitude is sensibly con- 

 stant, it proves that the True Aberration is the same as the Received Aber- 

 ration, or at least that one is not a multiple of the other. 



The last column of the Table is given only to show to how large an ex- 

 tent Aberration enters into the star's Apparent Declination. 



Every result for Observed Zenith-distance in the Table is the mean of 

 observations in reversed positions of the instrument. 



