XXI 



adjacent to them. Some idea of the labour thus expended may he gained 

 by the fact that for the two " Magellanic Clouds " 1 143 stars, nebulae, and 

 clusters were carefully measured, and 1203 stars for the great nebula in 

 Argo. Of the splendid monograph of this wonderful object which he has 

 given it is impossible to speak too highly, especially as, notwithstanding 

 some conflicting testimony, the comparison of it with recent observations 

 gives reason to believe that this nebula has undergone surprising changes 

 in the last thirty years. Independent of these precious catalogues the 

 volume is like a perfect gem ; besides the charm of its style, it is a rich 

 treasury of varied knowledge. As an example may be named the section 

 on the causes which injure the action of reflecting telescopes, the chapters 

 on astrometry, on Halley's comet, and on the solar spots ; they all bear 

 the stamp of a Master's hand, and contain suggestions all striking, and of 

 which some have since been applied with the best results to practice. His 

 labours on nebulas were completed much later in life by a general catalogue of 

 them containing 5079, which appeared in our 'Transactions' for 1864. This 

 includes all contained in his former catalogues and all discovered by others 

 up to that time. It contains the places for 1860, and their precessions for 

 1880 ; so that it will be easily available for observers to the end of the 

 century, and will long be their Manual of Nebulas. 



His contributions to optics rank next to his astronomical in importance 

 and number; but we shall only mention two, which gave a great impulse 

 to the progress among us of this branch of physics. The first is a remark- 

 able memoir on the aberrations of compound lenses and object-glasses, 

 which appeared in our ' Transactions' for 1821 . Before it opticians (at least 

 in this country) corrected the spherical aberration of their object-glasses 

 by empirical rules, and its theory was given in rude and unsymmetrical 

 forms. By a happy choice of symbols and an elegant analysis he presents 

 the theory of aberration in all its generality, and in as simple a manner as 

 the nature of the question admits. He gives examples of the application 

 of his theory to the construction of aplanatic doublets, and then to that of 

 object-glasses, In discussing this he considers the dispersive power as 

 composed of several terms, of which the first only is taken into account by 

 opticians, and the rest constitute the irrationality of the spectra. This 

 defect cannot be removed in a double object-glass unless the dispersive 

 powers of higher orders are proportional to the first ; and he recommends 

 that the attention of future inquirers should be directed to the discovery 

 of such a medium (a result which there is reason to believe has at last 

 been obtained by the combined investigation of the late \V. Vernon Ilar- 

 court and Professor Stokes). 



The condition which he assumes for correcting the spherical aberration 

 is, that the compound shall be aplanatic for near as well as remote objects. 

 He tabulates the results for the various crown and flint glasses then used 

 in England with a completeness which leaves nothing to be desired. It may 

 be feared, however (notwithstanding the popular explanation of his method 



