Limit to Defining -power. By J. J. Lister. 4^ 



thence to the middle of the second dark rin^r 



0000240 



a 



thence to the middle of the third dark ring 



0000241 



• — — « 



a 



These numbers are all less than Fraunhofer's, and the difference 

 between the semidiameter of the first ring and the space from it 

 to the second, which in his observations is J of the latter, is in 

 mine little more than ^q, and even this I have attributed to the 

 effect of the central spot upon the eye mentioned in p. 36, the 

 spaces in many of the less bright reflexions having been measured 

 as equal. 



When more minute globules give images too faint to show the 

 dark ring, the spots no longer retain their nearly uniform size but 

 are often much smaller. They are then, however, ill defined, and a 

 separation between two cannot be detected till their centres are as 

 far apart as the others. 



The angle '^^ , under which the patterns vanished when 



a 



viewed by the naked eye through apertures smaller than • 02 in., is 

 rather less than that inferred above as probable but differs little from 

 it, and the distinctness of the telescopic pencil (allowing for some 

 disadvantage from atmospheric faintness) and that of the Micro- 

 scope soon to be given, have appeared to go still somewhat further 

 under the most favourable circumstances ; so that I believe it will 



be little, if at all, beyond their true limit to take ", at • 000021 in. 



It will then follow that 6^ or the smallest angle under which a 

 separation in the parts of a pattern can be seen with a telescope, 



should be —, radius (d) being taken as unity, or 4*33 



a 

 seconds divided by the diameter in inches of the object-glass or 

 speculum. 



Also, since the diameter of the aperture divided by that of the 

 pencil entering the eye gives the magnifying power of the tele- 

 scope, we may expect to increase it to advantage on suitable 

 objects till this pencil is reduced to about 0*02 in. (see p. 39); but 

 beyond this the indistinctness increases as the pencil diminishes, 



so that j-^ should represent the highest magnifying power at 



which a telescope of the aperture a will give increase of details, 



