Final Bemarhs on Immersed Apertures. 223 



aperture its spherical aberration is of course no longer corrected for 

 uncovered objects." In measuring varying angles of aperture by 

 the usual method, we take them at all points of the adjusting collar, 

 and do not place in front a thickness of glass suitable for that cor- 

 rection, because with a parallel plate of glass there is no perceptible 

 difference. The angle at the crossing point of the rays is the same 

 whether it is there or not. I stipulate that the edges of the stop 

 shall be in the crossing point. If anyone thinks proper to intro- 

 duce an intervening j)late of glass, serving no purpose, he must 

 focus through it, so as still to get the stop in the focal plane. 

 Further, if the collar is set for an object immersed in balsam, for 

 the purpose of testing its reduced aperture therein by the means I 

 have described, the slit must be set in focus, whether air, water, or 

 balsam is the intermedium. In Mr. Tolles' ^th the immersed aper- 

 ture was found to be the same with all three, simply because they 

 are parallel plates. 



The question has now been so well ventilated that there is no 

 use in wearying readers with further theories and counter-theories, 

 perhaps only noticed by those engaged in the controversy, which 

 few care now to read. Anyone free from prejudice, and recognizing 

 the importance of cutting off all false rays within the focal point, 

 would use a suitable stoj) for the purpose, and throw all controversial 

 papers aside in favour of the practical proof in which the whole 

 question must culminate at last. All this voluminous correspond- 

 ence has arisen from a very small beginning, in which I pointed 

 out an optical error of Mr. Tolles in the direction of rays, and 

 which he unwisely chose to deny. In his last production of 180° 

 he has gone ahead of all others ; no one has surpassed him in that, 

 and perhaps argument will not be wanting to prove by diagrams 

 that he is right. 



There I leave him, not without some amusement at the gro- 

 tesque fatuity that induced his colleague to select the chaste and 

 Christianlike motto, " A blunder is worse than a crime !" * 



* See ' M. M. J.,' May, 1874, p. 228. 



VOL. XII. 



