"210 HERSCHEL. 



to admit " that a certain mecLauical effect, iujurious to clearness and 

 definition, would accompany the focal crossing of the rays of light."* 



This idea of the crossing of the rays suggested an experiment, 

 the result of which deserves to be recorded. A telescope of ten 

 English feet was directed toward 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 fine rigid 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 v/as 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 nnited. 

 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 this 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 experi- 

 ments, and yet does not entirely contradict them. In this instance, the 

 rays were of different origin, those coming from the advertisement and 

 those from the sun crossed each other, respectively, in an almost rectan- 

 gular direction; while in 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 results at which we 

 need to be much surprised. 



Herschel increased the catalogue, already so extensive, of the myste- 

 ries of vision, when he explained in what manner we must endeavor to 

 distinguish separately the two members of certain double stars very 

 close to each other. He said, "If you wish to assure yourself that tj 

 Corona) is a double star, first direct your telescope to a Geminorum, to 

 £: Aquarii, to p- Draconis, to p Herculis, to a Piscium, to e Lyrog. Look 

 at those stars for a long time, so as to acquire the habit of observing 

 such objects; then pass on to c Ursai majoris, where the closeness of 

 the two members is still greater. In a third essay select : Bootis, 



* Oa comparing the Cassegr.iin telescopes, with a small convex mirroi-, to the Gre- 

 gorian telescopes, with a small concave mirror, Captain Katcr found that the former, 

 in which the luminous rays do not cross each other before falling on the small mirror, 

 possess, as to intensity, a marked advantage over the latter, in which this crossing 

 talces place. 



