Astronomical Society. ' 4>55 



1 



" In many of the above cases it is probable that the difference be- 

 tween Piazzi's catalogue for 1800, and one reduced to that epoch from 

 Bessel's observations, may arise from error of observation : in others 

 the almost identity of the observations seems to prove at once the skill 

 of the observer, and a proper motion in the star j while again there 

 is one (No. 2.) which possibly may be regarded as conclusive on the 

 latter point. I would also suggest two instances of what appear 

 errors of the press, or perhaps in reading off the instrument, in M. 

 Bessel's zones. 



Z. 154. JR. 14 h 33 m 33-7." Dec. 12° 4P 22"-4 + Unless 26' be 

 read for 41' this star of the sixth magnitude, 32 Bootis, will 

 differ from Piazzi's catalogue +14' 56"-88. 



Z. 162. JR. 14 h 37 m 10 s -3.Dec. 9"° 55' 36"'8 + Unless 8° be read 

 for 9° this star of the 7*8 magnitude will differ from Piazzi 

 1° 0' 0"-39." 



April 11. — A paper was read " On the construction of large Achro- 

 matic Telescopes." By A. Rogers, Esq. — In this paper the author de- 

 scribes a new construction of an Achromatic Telescope, the object of 

 which is to render a small disc of flint glass available to perform the office 

 of compensation to a much larger one of crown, and thus to render 

 possible the construction of telescopes of much larger aperture than 

 are now common, without hindrance from the difficulty at present ex- 

 perienced in procuring large discs of flint glass. It is well known that 

 in the ordinary construction of an achromatic object-glass, in which 

 a single crown lens is compensated by a single one of flint, the two 

 lenses admit of being separated only by an interval too small to afford 

 any material advantage in diminishing the diameter of the flint lens, 

 by placing it in a narrower part of the cone of rays, the actual amount 

 of their difference in point of dispersive power being such as to 

 render the correction of the chromatic aberration impossible, when 

 their mutual distance exceeds a certain limit. 



This inconvenience Mr. Rogers proposes to obviate, and obtain 

 the advantage in question, by employing as a correcting lens, not a 

 single lens of flint, but a compound one consisting of a convex crown, 

 and concave flint, whose foci are such as to cause their combination 

 to act as a plane glass on the mean refrangible rays. Then it is 

 evident that by reason of the greater dispersive power of flint than 

 of crown glass, this will act as a concave on the violet, and as a con- 

 vex on the red rays ; and that, the more powerfully, according as the 

 lenses separately have greater powers or curvatures. If then, such a 

 compound lens be interposed between the object-glass of a telescope, 

 supposed to be a single lens of plate or crown glass, and its focus, it 

 will cause no alteration in the focus for mean rays, while it will 

 lengthen the focus for violet, and shorten it for red rays. Now this 

 is precisely what is wanted to produce an achromatic union of all the 

 rays in the focus ; and, as nothing in this construction limits the 

 powers of the individuals composing the correcting lenses, they may 

 therefore be applied any where, that convenience may dictate j and 

 thus, theoretically speaking, a disc of flint glass, however small, may 

 be made to correct the colour of one of crown, however large. 



But 





