40 BULLETIN OF THE 



3d. An objective is properly corrected for any given purpose 

 when its minimum focal distance corresponds to rays of the wave- 

 length which is most efficient for that purpose. For example : 

 in an objective corrected for visual purposes the rays which seem 

 brightest to the human eye should have the minimum focal dis- 

 tance; while in an objective intended for photographic work the 

 rays which produce the greatest effect upon silver bromo-iodide 

 should have the minimum focal distance. 



4th. In the case of a double achromatic objective, the second- 

 aiy spectrum (or in other words, the diameter, at its intersection 

 with the focal plane, of the cone of rays having the maximum 

 focal length) is absolutely independent both of the focal length 

 of the combination, and of the curves of its lenses; and depends 

 solely upon the aperture of the combination, and the physical 

 properties of the materials composing it. 



5th. When the focal curve of an objective is known; and the 

 relative intensity, for the purpose for which the objective is cor- 

 rected, of light of every wave-length, is also known; then the 

 exact position which the focal plane should occupy can be readily 

 calculated. 



Incidentally, it may be remarked that in an objective corrected 

 for photographic purposes the interval between the maximum and 

 minimum focal distance is less than in one corrected for visual 

 purposes. Hence a photographic objective has less secondary 

 spectrum, and is better adapted for spectroscopic work, than a 

 visual objective. 



Prof. A. Hall read a paper entitled 



NOTES ON THE ORBITS OF TITAN AND HYPERION. 



He stated that during the past wnnter he had collated and reduced 

 all observations of Hyperion (the seventh and faint satellite of 

 Saturn) that have been made since its discovery in 1848 by the 

 Bonds at Cambridge, and by Lassel in England. Tiie observa- 

 tions made in 1848 by the Bonds were not well adapted to the 

 determination of its orbit, since the plane of the orbit was seen, 

 edgewise. In 1852 the plane of the orbit having opened out, 

 Lassell made a good series of thirty observations, from which: 

 Prof. Hall computed a set of elements that fix the position of the 

 satellite in its orbit with a good degree of certainty for that epoch. 

 In the year 1875 a series of observations was begun with the 

 Washington 26-inch refractor and continued to the present time, 

 and comparing the elements deduced from them for the present 

 epoch with those of 1853 it is possible to determine the periodic 

 time of the satellite and the motion of its apsides. In order to 



