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11.— On the Trincijple of testing Ohject-glasses hy Miniatures of 
Illuminated Objects examined under the Mieroscoj)e, especially 
of Sun-lit Mercurial Globules; and on the Development of 
Eidola or False Images. By Dr. Koyston-Pigott, M.A., 
r.E.S., F.E.A.S., F.C.P.S., formerly Fellow of St. Peter's 
Collegej Cambridge, 
(Bead before the Eoyal Miceoscopical Society, March 3, 1875.) 
Plates XCIX., C, and CI. 
The method of examining miniatures as a means of testing objec- 
tives, as fully described in the ^ Transactions of the Eoyal Society/ 
offers some intrinsic advantages not attainable by the examination 
of objects placed immediately upon the stage. For want of this 
method, it has hitherto been impracticable to develope the splendid 
interference phenomena of sun-lit mercurial globules, which can 
only be perfectly seen by the means of magnified miniatures. 
Comparison of the enlarged miniature with its original is so 
facile and self-testing, that the merest tyro can at once declare 
whether the image shown when highly magnified agrees with the 
thing itself. For instance, if he detects a kind of white smoke or 
fog or coloured haloes about the bright points of the miniature, he 
knows that there must be error somewhere. On the contrary, all 
doubt as to quality and goodness of definition is completely barred 
out, when the enlarged image emerges sharp and clear and brilliant 
in all its details. This can only happen when all the rays, whether 
white or coloured, emanating from each single point, are made to 
converge again into one and the same point in the image. The 
white rays, if achromatic, but not aplanatic, create a white mist : 
the coloured rays, prismatic haloes. 
And this brings me to the standard principle of perfect defi- 
nition, viz. that the image of a given point shall not be a confusion 
of images of that point, but one single point. But to attain this 
DESCRIPTION OF PLATES XCIX., 0., AND CI. 
Fig. 1. — Shows the arrangement for illuminating artificially a mercurial globule 
by means of a prism : the miniature is formed in the focal plane of the 
observing object-glass. 
„ 2. — LM the focal plane of vision: aberrating rays intersect in this plane 
and produce the diffraction phenomena which assume their peculiar 
forms according to obliquity, kind of correction, and quality of the 
glasses. 
„ 3-14. — Show the elegant and delicate varieties of diffractions, in different 
stages of obliquity and correction. 
„ 15, 16, 17. — Display of the conic sections into which the phenomena arrange 
themselves. 
„ 18. — Apertures forming luminous disks and eidola out of focus. 
„ 19. — An exceedingly minute brilliant disk out of focus. 
,,20,21. — Wire gauze and its eidola. 
