^Zm^i.lJfc'riS'] Royal Microscopical Society. 299 
site standard for definition is realized than Nobert's lines and 
" lined diatoms " no great improyement will be made in the best 
object-glasses. T]ie writer however ventures to express the opinion 
that a new standard for high-power definition will be found in the 
minute structure of the Podura scale, which affords the most severe 
trial for the correction of residuary aberration with which he is 
acquainted. And having given much leisure to the use of this 
interesting object in estimating definition and the possibilities of 
improvement, he prefers it to all others. 
This extraordinary object, dating as a test from the jewel- 
microscopes of Pritchard and Dr. Goring to the splendid glasses of 
our eminent makers of the present day, has accomplished more 
towards the perfection of defining power than any other. It has 
done for the microscope what Sir William Herschel's close double 
stars and the rings and satelHtes of Saturn have done for the develop- 
ment of the charming and exquisite revelations of the telescopes of 
the time present. 
The American Government has lately authorized the exhibition 
of photographs of several microscopical objects, taken by means of 
Wales' and Powell and Lealand's glasses : these sun-pictures, 
especially those by Powell's |th, sVth, and sVth, agree remarkably 
with the accepted appearances beautifully delineated in Dr. Carpenter's 
work and that of Messrs. Smith and Beck. 
These photographs, taken by an American artist * and exhibited 
in England — might fairly be accepted as a challenge to English 
microscopists. But for this circumstance, the writer having awaited 
seven years for confirmatory evidence of his own results, now 
ventures to bring before the notice of the Society these observations : 
and in doing so he begs to remark that he believes they are capable 
of two kinds of demonstration, the synthetic and analytic, as far as 
the eye is concerned. But in order to obtain similar results, par- 
ticular attention must be paid to the following conditions : — 
(a.) Illuminating pencil. — A cone of light achromatic and 
aplanatic ; angle of apex about 20^ ; inclination of axis 
of cone to plane of stage about 20° to 30°. The size of 
the condensing lenses employed is of no consequence, the 
other conditions remaining the same. 
(b.) The scales which are darkest and smallest and with the 
longest diameter towards the light should be carefully 
chosen first with a good J-inch at 120 diameters, 
(c.) The most patient corrections should be applied as to the 
object-glass, chosen distance of the secondary focal image, 
and the lenses for the eye-pieces ; and lastly, the foci of 
the object-glasses and depth of the eye-pieces (so as to 
correct as much as possible the residuary aberration of 
the object-glass) should be most carefully selected. 
* Colonel Dr. WooJwarcl. 
