222 Royal Astronomical Society* 



It is right that I should now say soir.elliing of the performance 

 of the nine-inch and two-feet reflectors. And first, as regards the 

 success of the system of mounting adopted in securing the peenHar 

 advantages of the equatorial movement. This appears to have been 

 very complete. The measurements, both differential and micro- 

 metrical, made with them, and recorded in our Notices, show that 

 in this respect they may be considered on a par with refractors, and 

 in ficility of setting and handling they appear nowise inferior. Of 

 the optical power of the former, two facts will enable the meeting 

 to form a sufficient judgement. With this instrument Mr. Lassell, 

 independently and without previous knowledge of its existence, de- 

 tected the sixth star of the trapezium of 9 Orionis, And with this, 

 under a magnifying power of 450, and in very unfavourable circum- 

 stances of altitude, both himself and Mr. Dawes became satisfied of 

 the division of the exterior ring of Saturn into two distinct annuli, a 

 perfectly clear and satisfactory view of the division being obtained. 



The ieats performed by tlie larger instrument have been much 

 more remarkable and important. It has established the existence 

 of at least one of the four satellites of Uranus, which since their an- 

 nouncement by Sir W. Herschel had been seen by no other observer, 

 viz. the innermost of all the series, and afforded strong presumptive 

 evidence of the reality of another, intermediate between the most 

 conspicuous ones. The observations of M. Otto Struve, if they 

 really refer to the same satellite, are of nearly a month later date. 



'i'o Mr. Lnssell's observations with this telescope we also owe the 

 discovery of a satellite of Neptune. The first occasion on which 

 this body was seen was on the 10th of October, 1846, but owing to 

 the then rapid- approacli of the planet to the end of its visibility for 

 the season, it could not be satisfactorily followed until tlie next year, 

 when, on the 8th and 9th of July, observations decisive as to its 

 reality as a satellite were made, and in August and September full 

 confirmation was obtained. This important discovery has since 

 been verified both in Russia and in America. I call it so, because, 

 in fact, the mass of Neptune is a point of such moment, that it is 

 difficult to overrate the value of any means of definitively settling 

 it. Unfortunately, the exact measurement of the satellite's distance 

 from the planet is of such extreme difficulty, that up to the present 

 time astronomers are still considerably at issue as to the result. 



I come now to the most remarkable of Mr. Lassell's discoveries, 

 one of the most remarkable, indeed, as an insulated fact, which has 

 occurred in modern astronomy : though, indeed, it can hardly be 

 regarded as an insulated fact, when considered in all its relations. 

 I need hardly say that I allude to the discovery of an eighth satel- 

 lite of Saturn, a discovery the history of which is in the highest de- 

 gree creditable, not only to the increased power of the instruments 

 with which observatories are furnished in these latter days of astro- 

 nomy, but also to the vigilance of observers. IfT am right in the 

 principle that discovery consists in the certain knowledge of a new 

 fact or a new truth, a knowledge grounded on positive and tangible 



