Astronomical and Nautical Collections. 139 



lens, if the fringes are properly formed. It would cer- 

 tainly be possible to construct an apparatus for performing 

 this adjustment with facility, but great care would be required 

 in its execution ; and if the process which I have described 

 is somewhat more troublesome, it has at least the advantage 

 of requiring no other instrument than two small mirrors, 

 either of metal or of black glass, which any person may 

 easily procure. 



In this experiment, as in those upon diffraction, it is neces- 

 sary to employ the light of a single radiant point only ; and 

 in order that the fringes may be very distinct, it is necessary 

 that it should be the more minute or the more distant as 

 they are narrower. It is, however, of little consequence in 

 what direction the light of this point falls on the mirrors. 

 In order to see the fringes, we must go to a little distance 

 from the mirrors, and receive directly the rays which they 

 reflect on a lens of a short focus, behind which the eye must 

 be held in such a manner as to see all its surface illuminated. 

 The fringes will then be discoverable in the space illuminated 

 by both the mirrors, which is easily distinguished from the 

 neighbouring space by the greater brightness of the light. 



The fringes present a series of bright and dark stripes, 

 parallel to each other, and at equal distances. In white light, 

 when care is taken to make the fringes broad enough, they 

 are embellished by very vivid colours, especially towards the 

 middle of the space ; for at a distance from the middle the co- 

 lours gradually become fainter, and disappear altogether after 

 about eight alternations. In a more homogeneous light, such 

 as may be obtained by means of a prism, or of certain red 

 glasses, the number of fringes becomes much greater, each 

 of them, however, consisting only of dark and bright spaces 

 of the same colour : and by employing a light as purely 

 homogeneous as possible, the phenomenon is obtained in 

 the greatest possible simplicity ; and it is in this form that 

 it will be most convenient to study it in the first instance. 

 It will afterwards be easy for us to understand the appear- 

 ances which are observed in white light by the superposition 

 of bright and dark stripes of each of the species of coloured 

 rays that compose it. 



