^CSL!^}S^TislT] lioyal Microsco])ical Society. 295 
II. — On Hig/h-^oiver Definition : with illustrative Examples. By 
G. W. KoYSTON-PiGOTT, M.A., M.D. Cantab., late Fellow of 
St. Peter's College, Cambridge, M.E.C.P., F.E.A.S., F.K.M.S. 
{Read before the Eoyal Microscopical Society, Novemler 10, 18G9.) 
Plate XXXIII. 
It is well known that the smallest visual angle subtended by a 
minute spot capable of being appreciated by the eye varies very 
much with the observer, even after proper adjustments have been 
made for long or short sight. The writer once tried the following 
experiment in order to determine the minimum visual angle. 
Keceding from a pole, diameter 1\ inch, elevated on a rising ground 
against the sky, it vanished at a distance of 1150 yards. The pole 
was capped with a black ball, to show its locality after the shaft 
disappeared. The angle subtended by I5 inch at 1150 yards' 
distance is about six seconds of arc. This occurred in 1834. In 
youth the eye is in general more sensitive, but the late celebrated 
astronomer, Mr. Dawes, retained surpassing keenness of sight to 
the last. To such, " definition " is a comparatively easy task with 
lower powers than to general observers. At a distance of a thousand 
yards our volunteers are taught that the head of a man sinks between 
the shoulders, and an appreciable visual angle is given by the bull's- 
eye at this distance of about 2 J min. (2' • 29). These facts supply 
us with data for judging of the average power of definition. 
Now suppose thirty or forty bull's-eyes in a straight line, 
painted black, in contact upon a white wall. Though 2 ft. in dia- 
meter, at a distance of a thousand yards only a black line would 
appear ; the eye could not, unassisted, in general define the bull's- 
eyes of which it was composed, — or, in other words, definition of a 
line of round, black dots, each of about 2J minutes' visual angle, 
would fail. Perhaps if each centre were perforated with an inch 
aperture and illuminated from behind, their definition would be pos- 
DESCKIPTION OF THE PLATE ON HIGH-POWER DEFINITION. 
Figs. 1 and 2. — Examples of diffraction and intorference, producing- a very perfect 
resemblance of Test-scale Podura markings by the intersecting beaded 
ribbing of two superimposed azure blue scales. 
„ 3 and 4. — Sketch of Podura seen by direct condensed light and of longitu- 
dinal beading, with a few beads of the uuder-surface glimmering 
thiough the upper set. 
„ 5 to 8. — ^"Admh-ation Test-scale." The general beading shows itself iu 
a great variety of forms, according to the state of the illuminating- 
pencils and the focussing upon the upper and under set. 
„ 7.— The most perfect resolution hitherto attained by exceedingly patient 
and finely-adjusted glasses. The ribbing (6), Fig. 8, is hei-e distinctly 
represented by darker beads of the upper rou]caus; the under-set 
always appear brighter in colour. Isolated beads, free from collateral 
interference rays, assume a natural resplendence and focal point. 
„ 8. — a, b, c, d, e, difl'erent appearances of the test-scale under different con- 
ditions of illumination and vision. The ribbing {h) is preliminary to 
the transformation of the spikes of a into the rouleaus of e. 
