Apparent Magiiitude pf the hor'iZG7ital Moon. 24 1 



phasnomenon, he justly acknowledges that at diflferent times 

 the moon appears of very different magnitudes even in th« 

 same horizon, and occasionally of an extraordinary large 

 size, of which he is not able to give a satisfactory explana- 

 tion. — Smith's Optics, vol. i. p. G3, &c.; Remarks, p. 53. 



It is really astonishing that this phsenomenon should have 

 remained for more than 2000 years without an explanation 

 founded on a better principle than that of mere opinion. 



That the dimensions of the pupil of the eye alter by the 

 stimulus of light, is a truth well known ; and it is also true, 

 that the picture of the irioon formed upon the retina is not 

 permanent, but varies as the dimensions of the pupil vary. 



To demonstrate this property of the eye, let A C B, (fig. Z, 

 Plate VII.) represent a plano-convex lens; PA and RB two 

 pencils of white or compound rays of light, falling upon it 

 at the points A and B, in a direction parallel to its axis : 

 also, let Axv and Byv be the red or least refrangible rays, 

 and Aqy and Bc/x the violet or most refrangible. The red 

 ray from A will cut the violet ray from B at the point x, 

 and the red ray from B will cut the violet ray from A at the 

 pointy; through these intersections draw the line a'y, and 

 this line will be the diameter of the least circular space into 

 which all the rays that fall upon the lens parallel to its axis 

 can be collected ; and this circle, which for brevity's sake is 

 called the circle of aberration, is the true focus of the lens, 

 or place where the imag;e of the object is formed. 



Let the sine of incidence going out of glass be ?i, the 

 sines of refraction (into air) of the least and the most re- 

 frangible ravs be p and q; then if a plano-convex lens be 

 exposed with the plane side to the sun, the diameter of the 

 circle of aberration xij (or image of the sun formed of rays 

 of different refrangibility) is to the diameter of the lens AB 

 as q — p to q + p — ^fi* . 



From this given ratio of AB to xy it follows, that the 

 image of the sun within a telescope varies with the aperture 

 of the object glass. 



Sir Isaac Newton found, by most accurate experiments, 



* This theorem is well known to mathematicians. 



Vol. 24. No. 95. April 18()G. Q^ that 



