gaa a sm PROCEEDINGS OF SOCIETIES. BY | 
made by Mr. Wenham upon “ definition” in the last number of the 
Journal :— 
2, LanspowNE Crescent, W., 
June 8, 1870. 
Mr. Presipent,—Mr. Slack having referred at the last meeting to 
a “long evening,’ in which I had the gratification of exhibiting to 
him some novel experiments on the detection of and measurement of 
spherical aberration (and chromatic effects), I can only regret that 
the author of some recent remarks on a paper of mine “On High- 
power Definition” was not present on that occasion. 
The new testing consists of forming on the immersion lens focus 
of the microscope to be tested brilliant images of illuminated disks of 
light, diminished two thousand times in miniature, each disk being 
accurately formed by apertures ,32,ths of an inch in diameter 
punctured in blackened brass and broached carefully with Swiss tools. 
‘These apertures are arranged from the 20th, 15th, 10th, and 5th of an 
inch apart centre to centre. The closest pair appear on the stage of 
the microscope as images, each disk being zgtooth of an inch in 
diameter, and an aristokratic image of one disk would be (if the 
glasses were perfect) exactly j¢¢¢qath of an inch in diameter, or 
teooooth of an inch in round numbers. 
The closest pair are divided by an interval exactly 41th (four times 
nearly) the diameter of a disk. 
The finest objectives, such as I have possessed hitherto, scarcely 
divide this artificial star-doublet. The spurious disks of double stars 
appear swelled out so as to appear almost to coalesce. The lateral 
aberration in this case is nearly z5}5 th of an inch. 
Mr. Slack noticed that the correcting apparatus WIDELY DIVIDED 
this new double-star test for microscopic objectives. 
Very beautiful rings are visible symmetrically placed just within 
and without the best focal point, surrounding each of the star-disks 
forming the “close double.” 
In the case of the brilliant surface of a recently-broken piece of 
metal (and in exactly the same manner) every luminous point swells 
out to a spurious disk, so as to trespass on the images of its neigh- 
bours, and the innumerable assemblages of the spurious disks of resi- 
duary aberration (a subject upon which I propose to write a paper) 
renders the definition extremely difficult. 
Even if the surface of metal, such as steel, iron, copper, brass, 
silver, and gold, be very highly polished, I can readily develope the 
appearance of a complex structure apparently composed of an agglo- 
meration of spherules separated by dark lines; and the different 
qualities of polished iron, of which it is known many exist, are thus 
made evident to the eye of the observer, armed with glasses adequately 
corrected ; especially with an inch or half-inch employed with a power 
of 800 diameters. A much less power than this is inadequate. But 
the extreme closeness of the objective to the object under this power 
in general makes the observations impracticable with ordinary instru- 
ments, as already stated in the REMARKS. 
Taking so great an interest in my researches, you will be pleased 
