1862.] 269 



circles grow less till they vanish, that the distance of the points of 

 contact in the incumbent line was always equal to the distance of the 

 centres. "We should therefore be presented with rectilinear quadri- 

 lateral plane figures, of which, from the equality of opposite sides, and 

 the angles at the two extremities being right angles, all the angles 

 must be right angles. From which it would be an easy step to the 

 proof that the angles of every rectilinear triangle are together equal 

 to two right angles ; and so the Theory of Parallels be entered by 

 another door, and the twelfth axiom be a deduction instead of a 

 groundwork. 



It would be interesting if the Theory of Parallels should be proved 

 traceable to Plato's property of the sphere. 



X. " Letter to Professor STOKES, Sec. R.S., containing Observa- 

 tions made at Malta on a Planetary Nebula." By WILLIAM 



LASSELL, Esq., F.R.S. 



Malta, 26th Sept. 1862. 



MY DEAR SIR, In directing my large equatoreal upon the well- 

 known planetary nebula situated in JR. 20 h 56 m N.P.D. 101 56' 

 (1862), it has revealed so marvellous a conformation of this object 

 that I cannot forbear to send you a drawing of it, with some descrip- 

 tion of its appearance. With comparatively low powers, e. g. 231 



and 285, it appears at first sight as a vividly light-blue elliptic nebula, 

 with a slight prolongation of the nebula, or a very faint star, at or near 

 the ends of the transverse axis. In this aspect the nebula resembles 

 in form the planet Saturn when the ring is seen nearly edgewise. 

 Attentively viewing it with higher powers, magnifying respectively 

 760, 1060, and 1480 times, and under the most favourable circum- 

 stances which have presented themselves, I have discovered within the 

 nebula a brilliant elliptic ring, extremely well defined, and apparently 

 having no connexion with the surrounding nebula, which indeed has 

 the appearance of a gaseous or gauze-like envelope, scarcely inter- 

 fering with the sharpness of the ring, and only diminishing somewhat 

 its brightness. This nebulous envelope extends a little further from 



