318 Transactions of the Boyal Microscopical Society. 
II . — On the Visibility and Optical Aspects of Hairs vieived from 
a Distance. By Henry J. Slack, F.G.S., President li.M.S. 
{Read before the Eoyal Micrcjscopical Society, November 13, 1878.) 
Dr. Eoyston - Pigott’s observations on the visibility of minute 
objects suggested the following experiments. The first was to try 
at what distance persons of average good sight could see a human 
hair inch in diameter, and eight or nine inches long, fixed 
by a little gum at each end to a window-pane of plate glass. Five 
persons, three being ladies of different ages, all saw it distinctly at 
34 feet, against a whitish sky. By shifting the position of the 
observers the background could be varied with different sky tints, 
or by bringing rich green cypress trees into view. A white cloud 
as background gave the best result ; the dark trees answered well 
in some lights, and clear bluish sky considerably shortened the dis- 
tance of visibility. These trials were made with the hair on a 
northerly window, by observers looking at it from the north-east. 
The day was fine, and the time about 10.30 and II a.m. on the 
3rd November. Similar results were obtained on other days. 
Immediately after these experiments on the 3rd November, a 
similar hair was stretched vertically across a small pane of glass, 
which was raised about 10 feet on a pole, placed to the east, and 
with clear sky behind it ; the sun being within an hour or less of 
south. One of Browning’s Panergetic opera-glasses, with 2^ field 
lenses, and excellent corrections, magnifying rather less than 2, 
enabled one of the party to see the hair instantly at a distance of 
50 yards. The other four all saw it with the same instrument, 
but had to take great care, and succeeded best by steadying them- 
selves against a wall. The spectators all stood in the shade. 
It being obvious that a pane of glass behind the hairs might 
affect the vision, a frame was made of deal lath, with an opening 
5 inches square, and two hairs YoN6 diameter stretched 
across it at right angles to each other. This frame was fixed to a 
post, so that the crossed hairs could be seen in full face from either 
north or south. The time was 9th November, from 10.15 to 
II a.m. ; the sky being very clear and sun brilliant. Standing with 
our backs to the sun, both hairs were distinctly seen by my wife 
and self, at a distance of 29 feet, wuth a bluish grey sky for a 
background. When a sheet of white paper was placed behind the 
frame, the hairs \iere visible at nearly 46 feet (45 feet 6 inches). 
By this time the sun was rapidly gaining a position most favour- 
able to a brilliant illumination of both hairs, but especially the 
horizontal one. The hairs were then viewed from the north, and 
seen at a distance of 76 feet with a clear bluish sky background 
and naked eye. By a slight shift of position the sky background 
