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, 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 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, in 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 



