Monthly Mi ical , 
ee | of the Microscope. 267 
the double star, one-twentieth part of that diameter will be the 
residuary aberration of the microscope employed. 
It is perfectly clear that no searching for a fine definition among 
a host of refracting beading packed in one or more strata could ever 
reveal this simple truth of the double-star test. The superiority of 
the artificial test by known over natural tests of conjectural structure 
is here completely demonstrated. 
It would not be an honest solution of the difficulty to omit to 
mention that all single spherical surfaces, whether reflecting or 
refracting, are afflicted with aberration in its worst form when 
oblique reflexion or refraction is employed. The image even in 
the globule is necessarily coarsely defined, infinitesimal as it may 
seem. A single plano-convex lens, if it could be made infinitesi- 
mally small, would still be uncorrected, if spherically formed ; as 
no such single lens can form perfect test images. I have therefore 
endeavoured to produce images of the utmost perfection by suitable 
contrivances. 
Tue Imace Tzst. 
In a paper read before the Royal Society this year (not yet 
published) I attempted to explain that images formed in miniature 
by a finely corrected objective were remarkably free from aberra- 
tion, the residuary aberration being greatly diminished in the 
miniature image; and that a fine objective forms in its focus an 
image of marvellous beauty, when the object is placed at 10, or 
even 100 inches from it. Double stars formed near the apex of 
mercury globules, or by means of small given perforations in metal 
laced before a brilliant source of light, may thus be transferred to 
the stage of the microscope—avoiding all the errors and incon- 
veniences of the minute globule incorrectly illuminated by oblique 
light. 
ce variety of interesting objects artificially prepared display in 
this image all their details when examined under the microscope, 
with a perfection entirely due primarily to the quality of the 
microscope itself, and secondarily to the quality of the image- 
forming objective; the apertures of each instrument, the image- 
viewing and image-forming objective, being regulated to equality. 
The correction or examination of the residuary errors in the 
microscope, under the safe guidance of a perfect test image of a 
known object, is a new art. 
The reality of this kind of test often dispels the illusory per- | 
formance of “some wonderful quarter” upon some thing of beauty 
and a toy for ever to the admiring possessor. When perfect 
images cannot be seen, when a brilliant disk appears four times its 
