74 Immersion Lenses and New Befractometers. 



That the writer of the present article lias described several 

 appearances which Mr. Wenham has never yet acknowledged to 

 have been able to see, who therefore denounces them as spurious ! 



It was then pointed out that double stars could be imaged to 

 any degree of minuteness even 100,OOUth of an inch apart, upon a 

 minute mercurial globule, in reply to Mr. Wenham's challenge. 



^VbereujDon he roundly declares (January No., 1871) : — 



" I must protest against the positive manner in which Dr. 

 Pigott still totally condemns the mercury globule. This self- 

 confidence may be attributed to non-acquaintance with it." 



Next, speaking of my double-star image test, he rather oddly 

 admits : — 



"That the separation of two points of light on the globule 

 coming from two sources placed at a given interval is a test no 

 one will question." 



I am exceedingly glad to hear this. I was really afraid Mr. 

 Wenham would annihilate this experimentum crucis after the 

 solemn protest aforesaid. But Mr. W. is more generous still. He 

 grants me the whole pith of the question in one full admission, 

 which saves the writer from all further anxiety about Mr. W.'s 

 opmion. He magnanimously asserts : — 



" The greatest value of its indications are when it (the globule) 

 is considerably within or without the focus. . . . That it is em- 

 ployed solely for the purpose of obtaining a distinct image is quite 

 a mistake. . . . No optician thinks of the possibility of a distinct 

 image." 



If the reader will turn to page 265, November Journal, 1870, 

 he will find a statement of mine to the following efiect :— 



" The image of a flame reflected by the test globule is a round 

 disk, much larger than it ought to be, as seen in all isolated brilliant 

 points in the microscopical field. It is interesting to inquire at 

 what size its image can be discovered." (The reply is never!) 

 " And if such small globules are employed that the shape is gone, 

 and if the real diameter of the spurious disk is a test of the correc- 

 tion of the glasses, what becomes of the boasted accuracy of the 

 globule test, in which the size of the spurious disk is totally 

 neglected ? No one ever thinks, in testing microscopes, at all 

 about this spurious appearance. In telescopic testing, the most 

 essential part of the ordeal is the diameter of the spurious disk." 



Mr. W. now declares that the globule test is used principally 

 for correcting the combinations of the lenses in detail, beginning 

 with the back set first, in which excessive aberrations are purposely 

 introduced, and therefore not " to correct the least fault." 



So that its employment to test the real focal point and the size 

 of the spurious disk is, after all, exactly what has been stated. The 

 fact is, for the deep objectives, anything but a wild attempt to get a 



