30 ANNUAL EECORD OF SCIENCE AND INDUSTRY. 



is of great interest, as this plienomenon is manifest not only 

 in such observations, but in all cases where a telescope is 

 employed for determining the place of the centre of a body 

 by means of observations on its border or limb. The values 

 of the diameters of Venus and Mercury obtained micromet- 

 rically, under ordinary conditions, are always greater than 

 those obtained during a transit, and differ the more according 

 as the aperture of the telescope employed is greater or less. 

 Thus, during the transit of 1874, Mouchez obtained, with an 

 equatorial of eight inches' aperture, the diameter of Venus as 

 64.38" ; the observations of Main, at Greenwich, using a tele- 

 scope of six and a half inches, and Plummer, of Durham, with 

 about six inches' aperture, gave a diameter of 64.73". This 

 result comes from the fact that the imao:e of a luminous 

 point given by a telescope is not a point, but a disk sur- 

 rounded by rings alternately dark and bright, whose diame- 

 ter diminishes with an increase of aperture. The difference 

 of the diameters of Venus or Mercury, obtained by the same 

 instrument during a transit, and under ordinary conditions, 

 should be double, or nearly double, the diameter of the disk 

 of a star (a point) given by that instrument, or at least a 

 quantity of the same order. The observations of Tennant 

 and Main on Venus, and Plummer and Main on Mercury, 

 give 0.724" and 0.602" for this quantity, and theory based 

 on measures of Dawes and Foucault gives for the same 

 quantity 0.854". The practical effect of this is that the ob- 

 servations of the transit of Venus should be reduced to what 

 they would have been provided all the telescopes had the 

 same aperture (in the equipment of the American parties 

 this w^as provided for by making all the equatorials of the 

 same aperture, viz., five inches), and for this purpose Andre 

 seeks to determine what he calls the equation of instrument- 

 al diffraction. To show the effect of this, he gives the fol- 

 lowing observations of the first internal contact at St. Paul's 

 Island durino: the transit of Venus in 1874 : 



6 .5, January 17, 1876, 205. 



