444 Astronomical and Nautical Collections. 



show how exactly they concur in confirming the theory; 

 which indeed appears to be abundantly demonstrated by the 

 numerous and diversified facts which have been already 

 adduced in support of it. It will be sufficient to conclude 

 this extract of the Memoir on Diffraction with a detailed de- 

 scription of an important experiment of Mr. Arago, which 

 furnishes us with a method of determining the slightest dif- 

 ferences of the refractive powers of bodies, with a degree of 

 accuracy almost unlimited. 



We have seen that the fringes^ produced by two very nar- 

 row slits, are always placed symmetrically with regard to 

 a plane passing through the luminous point and the middle 

 of the interval between the slits, as long as the two pencils 

 of light which interfere have passed through the same me- 

 dium, for instance, the air, as happens in the ordinary arrange- 

 ment of the apparatus. But the result is different when 

 one of the pencils continues to pass through the air, and the 

 other has to be transmitted by a more refractive body, a 

 thin plate of mica, for example, or a piece of glass blown 

 very thin : the fringes are then displaced, and carried to- 

 wards the side on which the transparent substance is placed : 

 and if its thickness becomes at all considerable, they are 

 removed out of the enlightened space, and disappear altoge- 

 ther. This important experiment, which was first made by 

 Mr. Arago, may also be performed with the apparatus of the 

 two mirrors, if the plate be placed in the way of one of the 

 pencils, either before or after its reflection. 



Let us now see what inference may be drawn from this 

 remarkable fact, by the assistance of the principle of inter- 

 ferences. The light stripe in the middle is always derived, 

 as we have already seen, from the simultaneous arrival of 

 rays which have issued at the same moment from the lumi- 

 nous point ; consequently, in the common circumstances of 

 the experiment, they must have described paths exactly 

 equal, in order to arrive in the same time at the place of 

 meeting: but it is obvious that if they pass through mediums 

 in which light is not propagated with the same velocity, 

 that pencil, which has travelled the more slowly, will arrive 

 at the given point later than the other, and the point will 



