Testing Object-glasses. By Dr. Royston-Pigott. 155 
cerned very beautiful well-defined dark bars nearly parallel, with 
four rows of beads between each. The pattern is exquisite, and 
varies with a change of the screw-collar, but they look quite as 
real as the other structure. Here is a remarkable case of eidola. 
The images of the lower structure mix with those of the upper. 
These false images are best seen when the objects are overlaid at a 
slight obliquity. If they cross each other at a large angle, then 
these eidola change to large bright beading twice the natural size. 
The transformation of appearances under the microscope by the 
application of a series of powers in order of power and merit, is 
a subject now worthy of special interest and development. For 
instance, the various plates and engravings of objects still extant, 
accumulated within the last fifty years, would upon patient in- 
vestigation yield instructive results as to the causes of these 
optical misrepresentations, for such, with the extraordinary ad- 
vances made in the goodness of modern glasses, we are now 
compelled to pronounce them. 
I have the pleasure of recording here that just about four 
years ago I had the honour of first exhibiting to Mr. Slack many 
examples of eidola at my house. 
The diagrams accompanying this paper represent various forms 
of interference phenomena developed by sun-lit mercurial globules, 
miniatured either by a lj-inch Ross or a \ Ross. The sub-stage 
of the microscope employed has several movements, made by the 
writer about ten years ago, which enable the operator to manage 
the miniature as easily as an object on the stage, and also to give 
great obliquity. The examination of oblique pencils coming from 
the miniature, as to their conditions of achromatism or aplanatism, 
whether well or ill corrected, is thus rendered particularly easy 
and interesting. The figures given represent some of the many 
forms developed by different conditions of correction and obliquity. 
I should state that the Figures 3-14 were copied from sun-lit 
globules at my house by Mr. Hollick, of 135, Tufnell Park Road, 
Holloway, N. 
VOL. XIII. 
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