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played, though uncorrected, miniature pictures of marvellous 
beauty, bearing considerable amplification ; whilst a combina- 
tion of achromatic lenges corrected with all the resources of 
modern art, seemed capable of forming an exquisitely small 
image of any given object placed at a distance from it, the 
appearance of which, when examined by the microscope to be 
tested, could at once be verified by the object producing the 
miniature test. When suitable precautions are taken—such 
as (1) axial coincidence of the objectives ; (2) proper correc- 
tions for an “ uncovered ” or aérial, or for an aqueous image 
when immersion lenses are employed, and for the distance of 
the object from the image-forming objective,—these miniature 
test-images bear an extraordinary amount of amplification by 
the microscope, displaying at once the erroneous corrections. 
It was found convenient in general to use the image or 
miniature-forming objective of a deeper focus than the obsery- 
ing, generally one half. 
The following experiments were undertaken to elucidate 
the operation of this system of testing. The mechanical 
arrangements are shown by diagrams, figs. 1, la, Plate IX. 
Experiment 1.—Mainiature of a small thermometer, the ivory 
scale being graduated 24° to the inch. A power of 300 dia- 
meters, gained by a low eyepiece “ A” and the objective of 
one eighth focal length (made expressly for Podura beading- 
test), was applied to view the miniature formed by a one six- 
teenth objective of excellent quality; and the following 
appearances were carefully noted at the time of observation. 
Result.—The sparkle of light on the bulb of the instrument, 
the graduation, and the metallic thread with the glass tube 
are mvisible, obscured by a nebulous yellow fog which no 
objective adjustments are able to dissipate. Fig. 3, Plate IX 
(fig. 5 shows improving definition). 
In consequence of this unexpected discovery, regarding the 
quality of a “very fine” one eighth, it was returned to the 
opticians to their surprise for better compensation. It was 
then, after more accurate compensation by them, again sub- 
mitted to precisely the same testing conditions. 
New results.—Appearance of a slight nebulous yellow 
cloud through which could be distinctly seen the ivory scale 
finely graduated, the bulb sparkle, and even minute sepa- 
rated mercurial particles scattered within the glass stem 
fig. 7). 
The definition had been therefore decidedly reformed. 
Previously, however, to the alteration, experiments had been 
tried for the purpose of ascertaining whether a defective glass 
