fter their union at the focus, whereas the
concave lenses received the same rays before that union. When the
observer made use of a convex lens, the rays that went to the back of
the eye to form an image on the retina, had crossed each other before in
the air; but no crossing of this kind took place when the observer used
a concave lens. Holding the double advantage of this latter sort of lens
over the other, as quite proved, one would be inclined, like Herschel,
to admit, "that a certain mechanical effect, injurious to clearness and
definition, would accompany the focal crossing of the rays of
light."[20]

This idea of the crossing of the rays suggested an experiment to the
ingenious astronomer, the result of which deserves to be recorded.

A telescope of ten English feet was directed towards an advertisement
covered with very small printing, and placed at a sufficient distance.
The convex lens of the eye-piece was carried not by a tube properly so
called, but by four rigid fine wires placed at right angles. This
arrangement left the focus open in almost every direction. A concave
mirror was then placed so that it threw a very condensed image of the
sun laterally on the very spot where the image of the advertisement was
formed. The solar rays, after having crossed each other, finding nothing
on their route, went on and lost themselves in space. A screen, however,
allowed the rays to be intercepted at will before they united.

This done, having applied the eye to the eye-piece and directed all his
attention to the telescopic image of the advertisement, Herschel did not
perceive that the taking away and then replacing the screen made the
least change in the brightness or definition of the letters. It was
therefore of no consequence, in the one instance as well as in the
other, whether the immense quantity of solar rays crossed each other at
the very place where, _in another direction_, the rays united that
formed the image of the letters. I have marked in Italics the words that
especially show in what this curious experiment differs from the
previous experiments, and yet does not entirely contradict them. In this
instance the rays of various origin, those coming from the advertisement
and from the sun, crossed each other respectively in almost rectangular
directions; during the comparative examination of the stars with convex
and with concave eye-pieces, the rays that seemed to have a mutual
influence, had a common ori