ates 
. HIS EXPERIMENTS. | 281 
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, haying 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 there- 
fore 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, 7m another 
direction, the rays united that formed the image of the 
letters. I have marked in Italics the words that espe- 
cially show in what this curious experiment differs from 
the previous experiments, and yet does not entirely con- 
tradict 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 di- 
rections ; 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 origin 
and crossed each other at very acute angles. There 
seems to be nothing, then, in the difference of the results 
at which we need to be much surprised. 
Herschel increased the catalogue, already so extensive, 
of the mysteries of vision, when he explained in what 
