528 Prof. Lovering on the Velocity of Light 



and after Fizeau had studied the variation of the velocity of 

 light in running water according as the motions agree or differ 

 in direction, Foucault was emboldened to attempt a measure of 

 the absolute velocity of light by an experiment which could be 

 brought within the compass of a single room. I translate his 

 own account of the arrangements made for this purpose. 



"A pencil of solar light, reflected into a horizontal direction by a 

 heliostat, falls upon the micrometric mark, which consists of a series 

 of vertical lines distant from one another one-tenth of a millimetre. 

 This mark, which in the experiment is the real standard of measure, 

 has been divided very carefully by Froment. The rays which have 

 traversed this initial surface fall upon a plane rotating mirror at the 

 distance of a metre, where they suffer the first reflexion, which sends 

 them to a concave mirror at the distance of four metres. Between 

 these two mirrors, and as near as possible to the plane mirror, is 

 placed an object-glass, having in one of its conjugate foci the virtual 

 image of the mark, and in the other the surface of the concave mirror. 

 These conditions being fulfilled, the pencil of light, after traversing the 

 lens, forms an image of the mark on the surface of this concave mirror. 



"Thence the pencil is reflected a second time, in a direction just 

 oblique enough to avoid the rotating mirror, an image of which it 

 forms in the air at a certain distance. At this place a second con- 

 cave mirror is placed, facing so that the pencil, once more reflected, 

 returns to the neighbourhood of the first concave mirror, forming a 

 second image of the mark. This is taken up by a third concave 

 mirror, and so on to the formation of a last image of the mark on 

 the surface of the last concave mirror of an odd number. I have 

 been able to use five mirrors, which furnish a line twenty metres 

 long for the ray to travel. 



" The last of these mirrors, separated from the preceding one, which 

 faces it, by a distance of four metres (equal to its radius of curva- 

 ture), returns the pencil back on itself — a condition surely fulfilled 

 when the returning image and the original image on the last mirror 

 but one coalesce. Then we are sure that the pencil retraces its 

 steps, returns in full to the plane mirror, and all the rays go back 

 through the mark, point for point, as they went forth. 



" This return of the pencil may be proved on an accessible image 

 by reflecting the pencil to one side by a surface of glass at an angle 

 of 45°, and examining it through a microscope of small power. The 

 latter, resembling in all respects the micrometric microscopes in use 

 for astronomical observations, forms, with the mark and the inclined 

 glass, one solid piece of apparatus. 



"The real image sent into the microscope, and formed by the 

 returning rays partially reflected, occupies a definite position in rela- 

 tion to the glass and the mark itself. This position is precisely that 

 of the virtual image of the mark seen by reflexion in the glass. At 

 least this is true when the plane rotating mirror is at rest. But 

 when this mirror turns, the image changes its place ; for while the 

 light is going and returning between the mirrors, the plane mirror 



