52 ABERRATION of the STARS, S:c. 



into the centre of the tube at top will not fa.ll upon the centre 

 at the bottom, unlefs the tube fliould be inclined towards the 

 moving particles like the tube i>, which inclination muft be more 

 or lefs as the velocity of the tube in croffing the diretftion of the 

 particles, is more or lefs fenfible when applied to their velocity. 

 Now fuppofe thefe particles to be rays of light, iffuing from a 

 itar, the line DC a portion of the Earth's orbit, and the tube a a 

 telefcope, then from the theory it is manifeft, that if the velocity 

 of the Earth in its orbit, bears a fenfible proportion to the 

 velocity of light, the telefcope muft have a direction which wiil 

 vary from the true place of the ftar, in order to bring the light 

 through the vilual axis of the inflrument. 



From the ratio of the velocity of the Earth in its orbit, to 

 the velocity of light, a flar may poffibly appear 20" from its 

 true place, which has alfo been confirmed by celeftial obfer- 

 vation, and is the full aberration ; but this quantity in decli- 

 nation, and right afcenfion, will only be had in ftars particular- 

 ly fituated, as in the poles of the ecliptic for declination, and 

 in the folftitial colures for right afcenfion. A flar fituated in 

 either pole of the ecliptic, will apparently defcribe a circle 

 round its true place, whofe radius is 20" ; and in the ecliptic 

 apparently vibrate backward and forward in its plane, in a 

 flraight line whofe length is 40". In whatever figure the eclip- 

 tic would be proie£ted when viewed from a flar, that flar will 

 apparently defcribe a fimilar one, which mufl be either a 

 ftraight line a circle, or an ellipfe. — A flraight line if the flar 

 is in the ecliptic, a circle if in either pole of the ecliptic, and if 

 in either of the intermediate fpaces an ellipfe, whofe femitranf- 

 verfe will be" 20', and femi-conjugate the fine of the flar's lati- 

 tude, making radius, or the fine of 90" equal to 20. fo far 



for the theory. 



It will be advifible for thofe not conflantly in the habit of 

 making the calculations, to begin by projefting the cafe, which 

 may be done as follows. For an example take /i Medufa?, 

 whofe longitude is i* 23° 13', and latitude 22° 28' north. — 

 From any fcale of equal parts take 20, and with that extent 

 for a radius defcribe the circle ABCD, (Fig. 2 Plate I.) through 

 which at right angles to each other, draw the diameters AC, 

 and BD : let BD be the tranfverfe diameter of the ellipfe. 

 Then for the conjugate fay 



4' As 



