36 The Astronomer Royal on a supposed Alteration [Nov. 23, 



angular displacement of a star or planet which is caused by the Earth's 

 motion of translation, and is known as the Aberration of Light. It has been 

 conceived that there may be a difference in the amounts of this displace- 

 ment, as seen with different telescopes, depending on the difference in the 

 thicknesses of their object-glasses. The most important of the papers con- 

 taining this discussion are : — that of Professor Klinkerfues, contained in a 

 pamphlet published at Leipzig in 1867, August; and those of M. Hoek, 

 one published 1867, October, in No. 1669 of the ' Astronomische Nach- 

 richten,' and the other published in 1869 in a communication to tbe Ne- 

 therlands Royal Academy of Sciences. Professor Klinkerfues maintained 

 that, as a necessary result of the Undulatory Theory, the amount of 

 Aberration would be increased, in accordance with a formula which 

 he has given ; and he supported it by the following experiment : — 



In the telescope of a transit-instrument, whose focal length was about 

 18 inches, was inserted a column of water 8 inches in length, carried in a 

 tube whose ends were closed with glass plates ; and with this instrument 

 he observed the transit of the Sun, and the transits of certain stars whose 

 north-polar distances were nearly the same as that of the Sun, and which 

 passed the meridian nearly at midnight. In these relative positions, the 

 difference between the Apparent Right Ascension of the Sun and those of the 

 stars is affected by double the coefficient of Aberration ; and the merely 

 astronomical circumstances are extremely favourable for the accurate test- 

 ing of the theory. Professor Klinkerfues had computed that the effect of 

 the 8-inch column of water and of a prism in the interior of the telescope 

 would be to increase the coefficient of Aberration by eight seconds of 

 arc. The observation appeared to show that the Aberration was really 

 increased by 7"'l. It does not appear that this observation was repeated. 



A result of physical character so important, and resting on the respect- 

 able authority of Professor Klinkerfues, merited and indeed required fur- 

 ther examination. Having carefully considered the astronomical means 

 which would be most accurately employed for the experiment, I decided on 

 adopting a vertical telescope, the subject of observation being the meri- 

 dional zenith distance of y Draconis, the same star by which the existence 

 and laws of Aberration were first established. The position of this star is 

 at present somewhat more favourable than it was in the time of Bradley, 

 its mean zenith-distance north at the Royal Observatory being about 100" 

 and still slowly diminishing. With the sanction of the Government, there* 

 fore, I planned an instrument, of which the essential partis, that the whole 

 tube, from the lower surface of the object-glass to a plane glass closing the 

 lower end of the tube, is filled with water, the length of the column of water 

 being 35*3 inches. The curvatures of the surfaces of the two lenses con- 

 stituting the object-glass, adapted, in conjunction with the water, to correct 

 spherical and chromatic aberration, were investigated by myself and verified 

 by my friend Mr. Stone (now Astronomer at the Cape Observatory). The 



