592 DR. G. W. ROYSTON-PIGOTT OX A SEARCHER EOR APLANATIC IMAGES. 
expressly for this resolution; as this totally failed, a one-sixteenth was carefully con- 
structed with no better success, and finally a one-fourth of very large aperture ; all 
these failed to exhibit the Podura beading*. Some unsuspected cause of this failure 
evidently remained to be investigated. The evidently delusive character of the standard 
test, so much relied upon for the construction of microscopic object-glasses, suggested 
the necessity of a search for other less uncertain methods of testing. The principle of 
proceeding from the known to the unknown appeared to offer the only sound basis of 
inquiry. 
Simple objects were now examined. The finest glass threads presented linear images 
of any conceivable degree of proximity, whilst their fused extremities, when selected as 
forming refracting spherules one-thousandth of an inch in diameter, presented miniature 
landscapes and points of light of remarkable precision, the spherical aberration of which 
could be easily calculated to be of insignificant amount for limited apertures. Even a 
plano-convex lens of one-thirtieth of an inch focal length and three-hundredths aperture 
displayed, though uncorrected, miniature pictures of marvellous beauty, bearing consi- 
derable amplification ; whilst a combination of achromatic lenses 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 minia- 
ture test. When suitable precautions are taken — such as (1) axial coincidence of the 
objectives ; (2) proper corrections for an “ uncovered ” or aerial, 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 ampli- 
fication by the microscope, displaying at once the erroneous corrections. I have found 
it convenient in general to use the image or miniature- forming objective of a deeper 
focus than the observing, generally one-half. 
The following experiments, as elucidating the operation of this testing, will, it is hoped, 
explain its critical powers. The mechanical arrangements are shown by diagrams, figs. 
1, la, Plate LII. 
Experiment 1. — Miniature of a small thermometer, the ivory scale being graduated 
24° to the inch. A power of 300 diameters, 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-sixteenth 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 within the glass tube are invisible, obscured by a nebulous yellow fog 
which no objective adjustments are able to dissipate. Fig. 3, Plate LII. (fig. 5 shows 
improving definition). 
* Messrs. Powell and Lealand spared neither skill nor time in endeavouring to assist my inquiry ; but with 
these glasses I signally failed to exhibit to them the new test (1864). 
