446 Astronomical and Nautical Collections. 



tion is propagated along the fluid, but merely on the duration 

 of the previous oscillation which gave it birth ; consequently, 

 when the luminous waves pass from one medium into another, 

 in which they are propagated more slowly, each undulation 

 is performed in the same interval of time as before, and the 

 greater density of the medium has no other effect than that 

 of diminishing the length of the undulation, in the same 

 proportion as the velocity of light is diminished : for the 

 length of the undulation is equal to the space that the first 

 agitation describes during the time of a complete oscillation. 

 We may therefore calculate the relative velocities of light in 

 different mediums, by comparing the length of the undula- 

 tions of the same kind of light in those mediums. Now, the 

 middle of the central stripe is formed by the reunion of such 

 rays of the two pencils as have performed the same number 

 of undulations, in their way from the luminous point, what- 

 ever may be the nature of the mediums transmitting the 

 light. If then the central stripe is brought towards the 

 side of the pencil which has passed through the glass, it is 

 because the undulations of light are shorter within the glass 

 than in the air; and it is necessary, in consequence, that the 

 path described on this side should be shorter than the other, 

 in order that the number of undulations may remain the 

 same. Let us suppose, then, that the central stripe has been 

 displaced to the extent of twenty breadths of fringes, for 

 example, or of twenty times the interval between the middle 

 points of two consecutive dark stripes ; we must necessarily 

 conclude that the interposition of the plate of glass has re- 

 tarded the progress of the pencil passing through it to the 

 extent of twenty undulations; or that it has performed within 

 the plate twenty undulations more than the same pencil would 

 have performed in an equal thickness of air, since each breadth 

 of a fringe answers to the difference of a single undulation. 

 If then we know the thickness of the plate, and the length 

 of an undulation of the light employed, which is easily de- 

 duced from the measurement of the fringes, by the formula 

 that has been given, we can calculate the number of undu- 

 lations comprehended in the same thickness of air, and by 

 adding twenty to the number, we shall have that of the un- 



