OBITUARY NOTICE OF JOSEPH JACKSON LISTER 547 



tables that the performance of each combination is given, both when the glasses 

 are close and when they are separated a certain distance from each other. As 

 we look down the tables we seem for a while to find confusion worse confounded. 

 We see, indeed, abundant evidence of the great effect produced both upon coma 

 and upon spherical aberration by the distance between the glasses ; but the 

 effects appear altogether inconsistent, if not contradictory. Thus, as regards 

 coma, two of Fraiinhofer's glasses which, if used singly, gave slight outward 

 coma, gave when combined and near together a great deal of coma rather outward, 

 but when separated b}^ 12 inches an almost entire absence of coma, and what 

 there was rather inwards. But, farther down, three glasses which each ga\" 

 outward coma when single, are seen to present in combination an inward conia 

 when close, and an outward coma when separated. With respect to spherical 

 aberration we seem for a while to meet with something like a law. We find 

 that two glasses which, if used alone, are free from spherical error, when com- 

 bined and close have that error over-corrected, but this over-correction is re- 

 moved by separating the glasses. And the same thing occurs with se\'eral other 

 combinations. But looking down the table we come to a case where the excess 

 of spherical correction caused by a combination of three glasses placed close, 

 cannot be removed by separating them, and then follows a combination of 

 three, in which ' the excess of spherical correction is increased by separating 

 for the short distance we can go'. And, again, a little lower occurs a com- 

 bination, also of three, in which ' the excess of spherical correction is diminished 

 but not conquered ' by separation of the glasses. 



Yet out of this a])parent confusion he educed a principle which reconciled 

 all the conflicting appearances, and formed the basis upon which all tine com- 

 binations for high powers of the microscope have rested. He found that in 

 a plano-convex lens, constructed like those above described, in which a double 

 convex of plate has its colour corrected for a moderate aperture by a plano- 

 concave of flint, the effect of the flint lens upon the spherical error caused by 

 the plate varies remarkably according to the distance of tlie luminous point 

 from the glass. If the radiant is at a considerable distance, the rays proceeding 

 from it have their spherical error under-corrected ; but as the source of light 

 is brought nearer to the glass, the flint lens produces greater proportionate 

 effect, and the under-correction diminishes till at length a point is reached where 

 it disappears entirely, the rays being all brought to one point at the conjugate 

 focus of the lens. This, then, is an aplanatic focus. It {\\c himinous jioini 

 is brought still nearer to the glass, the influence of the flint lens continues for 

 a while to increase, and {\\c ojiposite condition, of o\-er-correction. shows itself ; 

 but on still furtlier api)ro\inKitic»n of the radiant, in eonsec|uenee apjxirenth' 



