178 PROCEEDINQS OF SOCIETIES. 



Society cuuld be involved. It is well known that from the first he 

 held the idea that these so-called spurious spines were a reality, and 

 he was now able to show a specimen in which one of the " spurious " (?) 

 spines was a detached body, which had been fairly dissected and cut 

 from the scale. "With respect to the correction of the object-glasses 

 for aberrations of covering glass, or differences of conjugate foci, he 

 agreed with Dr. Pigott that a correction might be obtained in the 

 way described. If they began with a short length of tube, for every 

 inch they added, there would be a proportionate alteration required 

 in the adjusting collar, and therefore a position might be found for the 

 eye-piece that would correct aberrations apparent in another position. 

 But he disagreed with the statement that the aben-ations which were 

 effected by an adjusting collar were in any way due to an extra number 

 of rays introduced, and caused by mere increase of aperture only. 

 These aberrations were not due to exterior rays, and it was quite 

 certain that in any case, whether the aperture were large or small, the 

 same series of the effects of chromatic aberrations would be found to 

 occur. Taking the light point from a very minute mercury globule in 

 the true focus of the object-glass, which is the real point from which 

 all the corrections must be made, and viewing the sections of the light 

 cone both within and without that focus, the indications would be the 

 same, whether the aperture were made larger or smaller by stops ; 

 that is to say, in either case there would be under-correction by 

 approximating the lenses, and over-correction by separating them. 



Dr. Pigott said he would not trouble the meeting at that late hour 

 with any lengthened remarks. He was sorry that Mr. Wenham did 

 not agree with the simple principle he had laid down. He believed 

 it was quite correct. He knew of no other form in which to express 

 it than as the coefficient of y square (as given in treatises on Optics). 



The President suggested that it was only spherical aberration to 

 which Dr. Pigott's remarks applied. 



Dr. Pigott said he was now alluding to spherical aberration. He 

 was prepared to maintain that the linear aperture of the pencil, 

 passing through the object-glass, regulated the spherical aberration. 

 If they moved the screw-collar, by so doing they altered the linear 

 aperture of the pencil ; therefore they altered the aberration which 

 varies as the square of the linear aperture. 



The President said this was perfectly true, but it should be borne 

 in mind that the alteration of the screw-collar did another thing, it 

 also altered the position of the lenses ; alteration of the aberi'ation 

 depended also upon the passage of rays through the combination. 



Dr. Pigott admitted that he was not tightly bound to the angular 

 aperture as the only explanation, though he thought it might be 

 useful to explain how the alteration in aberration was really effected 

 by a movement of the screw-collar. 



Mr. Wenham said that his remarks were based not upon any 

 mathematical formula or demonstration, but were simply taken upon 

 the usual practical method of measurement, with an artificial star 

 viewed within and without the focus proper, as the indications from 



