136 Astronomical and Nautical Collections, 



the fringes, following the steps by which it occurred 

 to me. 



Wishing to observe the external fringes very near the 

 opaque body, I attempted to receive the shadow on a plate 

 of ground glass, and to look at them from behind with a lens : 

 and in moving my eye till it had passed beyond the glass, I 

 remarked that they remained equally visible, and even more 

 distinctly, and that they were precisely similar to those 

 which appeared on the glass. Hence I inferred that the 

 glass was altogether superfluous, and that it was sufficient 

 to receive the light directly on the lens, looking at the lumi- 

 nous point from behind. The reason of this is very obvious: 

 the effect of a convex lens is to give a clear picture to the 

 eye of whatever is its focus, whether it may be a real object 

 or an image formed by any arrangement of rays of light; pro- 

 vided that these rays fall on the surface of the lens without 

 alteration. It is thus that the eye-glass of a telescope en- 

 ables us to see the aerial image of the objects depicted in 

 the focus of the object-glass ; an image which is seen in the 

 same form, though much less distinctly, when it is received on 

 white paper or on ground glass. This mode of observation, 

 which might have been suggested by theory only, is greatly 

 preferable to that which bad been commonly followed, 

 because it has the advantage of magnifying the fringes, and 

 of increasing their brilliancy at the same time ; which per- 

 mits us to distinguish them in a multitude of cases, when 

 they could not be distinguished on a piece of a white 

 paper, on account of their minuteness, or the weakness of 

 the hght. In order to see the fringes very distinctly, it is 

 necessary to take care that the focus of the rays converging 

 from the lens should fall on the middle of the pupil, by 

 placing the eye at such a distance from it, that all its surface 

 may appear illuminated when it is wholly exposed to the 

 light, and keeping the distance unchanged when the lens is 

 moved towards the shadow. It must also be observed, that 

 the margin of the wire must not be exactly in the focus of 

 the lens, since then the fringes disappear, for reasons which 

 will be easily understood hereafter. 



In order to establish the superiority of this mode of ob- 



