upon the Velocity with which it is traversed by Light, 247 
versely proportional to the refracting index. Arago and Fresnel 
have both shown the extraordinary sensitiveness of this method 
by several yery delicate observations, such as that on the difference 
of refraction between dry and moist air, 
_ A method of observation founded upon this principle appeared 
to me to be the only one capable of rendering evident any change 
of velocity due to motion, It consists in obtaining interference 
bands by means of two rays of light after their passage through 
two parallel tubes, through which air or water can be made to 
flow with great velocity in opposite directions, The especial 
object before me necessitated several new arrangements, which I 
proceed to indicate. 
With respect to the intensity of light, formidable difficulties 
had necessarily to be encountered. The tubes, which were of 
glass and 5:3 millims. in diameter, had to be traversed by light 
along their centres, and not near their sides ; the two slits, there- 
fore, had to be placed much further apart than is ordinarily the 
case, on which account the light would, in the absence of a spe- 
cial contrivance, have been very feeble at the pot where the 
interference bands are produced. 
This inconvenience was made to disappear by placing a con- 
yergent lens behind the two slits; the bands were then observed 
at the point of concourse of the two rays, where the intensity of 
light was very considerable, 
The length of the tubes being tolerably great, 1-487 metre, 
it was to be feared that some difference of temperature or pres- 
sure between the two tubes might give rise to a considerable 
displacement of the bands, and thus completely mask the dis- 
placement due to motion. 
This difficulty was avoided by causing the two rays to return 
towards the tubes by means of a telescope carrying a mirror at 
its focus, In this manner each ray is obliged to traverse the two 
tubes successively, so that the two rays having travelled over 
exactly the same path, but in opposite directions, any effect due 
to difference of pressure or temperature must necessarily be eli- 
minated by compensation. By means of various tests I assured 
myself that this compensation was complete, and that what- 
ever change in the temperature or density of the medium 
might be produced in a single tube, the bands would preserve 
exactly the same position. According to this arrangement, the 
bands had to be obseryed at the point of departure itself of the 
rays : solar light was admitted laterally, and was directed towards 
the tubes by means of reflexion from a transparent mirror ; after 
their double journey through the tubes, the rays returned and 
traversed the mirror béfore reaching the place of interference, 
where the bands were observed by means of a gery eye-piece,: 
2 
