On the Visibility, &c., of Hairs. By Henry J. Slack. 319 
was exchanged for a white wall, and the hairs were seen at a 
distance of 113 feet. Just about eleven o’clock the sun lit up the 
horizontal hair with a brilliant glow, and then an image, formed by 
it, was distinctly seen at the great distance of 173 feet, but the 
appearance was not at all hair-like. It was far too broad, and not 
sharp at the edges. The opera-glass did not make it look like a 
hair, but left no doubt that the hair caused the image, and that the 
image corresponded with it in position and length. A pencil was 
then held over the horizontal hair and seen with the opera-glass, 
without distortion. As soon as the sun’s motion carried it from the 
most favourable position for lighting up the hair, it ceased to give 
any image visible at a long distance. 
In experiments with hairs attached to a window-pane, it will be 
found that the quantity and direction of the sunlight, and the 
colour of the background, and its degree of illumination, all affect 
the apparent diameter of the hairs. A diffused rain-cloud with a 
good deal of white light in it, gives a small diameter, and very 
sharp definition. Taking this as the most correct aspect of the 
object, any cliange of conditions which makes it look broader must 
be regarded as introducing optical errors, and it may be remarked 
that when, after viewing a hair two or three feet off, it is viewed at 
15, 20, or 30 feet off, it does not seem to have its diameter 
diminished, as would result from perfectly accurate vision. 
So long as the image seen by the eye corresponds tolerably 
well with the true aspect of a hair, it is fair to say, “I see the 
hair ; ” but when such aspects are afforded as in the experiments 
with full solar illumination the hair is not seen, but only the effect 
of its reflexions and refractions on the solar beams. 
If an observer, seeing this appearance, concluded that the 
frame had a band across it in a central position, he would be right ; 
but wrong if he attempted to estimate its breadth, or to decide 
upon the nature of the substance composing it. This is a case of 
a kind of vision giving some information that corresponds with 
fact, coupled with other information that differs widely from fact ; 
and an inquiry suggests itself whether a good deal of vision 
obtained with high powers and peculiar illumination does not 
partake of this character. 
On the 12th November, the sunshine being brilliant and air 
clear, the experiments were repeated. At 10 a.m. both hairs were 
sharply seen at 105 feet, without distortion. At 10.12 the 
horizontal one was seen at 110 feet, and at about 115 feet twelve 
minutes later. The sky then became hazy, aod the hairs invisible 
at the long distances. 
About four o’clock on the 11th November, when walking up a 
hill two or three miles south-west of East Grinstead, the vanes 
standing above the angles of the four square towers caught the 
