ESSAY ON THE VELOCITY OF LIGHT. 165 
and formed of eleven platinum to the millimetre. Passing through this reticule, 
the beam of light meets an objective of a focus of two metres placed at a dis- 
tance from the reticule less than the double of its principal focal length, and it 
tends to form beyond a magnified image of the reticule. But before the forma- 
tion of this image the converging pencil is reflected from a small mirror which, 
capable of rapidly revolving around a vertical axis, we will call the revolving 
mirror. After its reflection, the converging beam will form an image before the 
mirror at a distance of 4 metres, and when the mirror turns, this image moves 
in space, describing circles double of the number of the turns of the mirror 
supposed to-reflect from its two faces. In sweeping through space this image 
meets a concave mirror whose centre of curvature corresponds with the centre 
of figure of the revolving mirror and with the centre of the axis of rotation; 
it thence results that during all the time that the image of the reticule falls on 
the concave mirror the light is thrown back to its point of departure by the 
revolving mirror and returns to form at the reticule its image of natural size. 
This image coincides exactly with the reticule, when the revolving mirror being 
at rest is placed at the proper angle of incidence; but as soon as it moves, the 
image is deviated and deflected in the direction of the motion. In order conve- 
niently to observe this deviation we place obliquely to the path of the entering 
beam a glass plate which throws this image to one side. ‘This image appears like 
colorless diffraction bands, striped with vertical lines, distant from each other 
the eleventh of a millimetre; they are examined with a powerful ocular, 
having at its focus a micrometer divided into tenths of a millimetre. ‘The 
stripes of the image bear the relation to the divisions of the micrometer as a 
scale to its vernier, so that deviations to the one-hundredth of a millimetre can 
be read off. Calculation shows that a deviation should be observed for thirty 
turns of the mirror in a second; and in fact it is seen for that velocity ; for 
_greater velocities the deviation is measurable. If we wish to measure the 
velocity of light in water we place between the revolving mirror and the con- 
cave mirror a tube three metres long, filled with perfectly clear water, and its 
ends closed by plates of glass of parallel surfaces. All things remaining the 
same, the deviation observed when we interpose the tube of water is always 
. greater than when this tube is not placed between the revolving and the con- 
eave mirror. But it is better, to operate simultaneously in the air and in the 
_ water, to employ two concave mirrors of the same radius of curvature and both 
facing the revolving mirror; one destined to receive and send the rays through 
the water, and the other through the air only. The mirror in revolving causes 
the two images, corresponding to the two reflections, alternately to appear, but 
the rapid succession of their apparitions makes them appear superposed ; to 
distinguish them from each other we cover a good part of the height of the 
concave mirror which reflects the image through the air, which reduces the light 
of the brighter image; the remainder of the field is occupied by the image 
which has traversed the water. The vertical stripes of these two images should 
_ correspond, and indeed do correspond, for low velocities of the revolving mirror. 
~ But as the velocity of rotation increases, the two rays are deflected unequally, 
the stripes break at the line of junction, and the deviation is greater for the 
' dull and green image which has traversed the water than for the luminous and 
~ white image which has progressed only through the air. This last experiment, 
although difficult to repeat with apparatus improvised in a hurry, has the ad- 
“vantage to appeal directly to the eyes; it has been repeated before several dis- 
“tinguished savants, who, in reference to it, no longer retain the least doubt. 
‘lo give to the mirrors rapid and constant velocities M. Foucault uses a 
“small steam-turbine, which was constructed with the greatest care by M. Iro- 
_ ment. We cannot at present enter into the details of its construction. It wil) 
_ be noticed hereafter, as well as the applications of this new method of expevri- 
menting. when the paper in which it is described has been presented to the 
Academy of Sciences. 
