J. Lovering on ies . Light and the Sun’s Distance. ey: 
“A pencil of solar light, reflected into a open et direction ey a he- 
Hostat, falls upon the micrometric mark, which consists of a series of ver- 
tical lines: distant from one another one-tenth of a millimetre. This 
mark, which in the experiment is the real standard of measure, has been 
div ided very sarebalty by F aps nie The rays, which have traversed this 
initial, surface, fall upon a plane rotating mirror at the distance of a 
metre, where. they suffer pete 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 Gian having 
in one of its conjugate foci - virtual image of the mark, and in the 
other the surface of the concave mirror. These ete being ful- 
filled, the pencil of light, ae wavereing 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 ob- 
lique enough to a void the rotating mirror, an image of which it forms 
is placed, facing so that the pencil, once more oe returns to the 
“The = of these mirrors, a er — the © pieced one, which 
returning _— and tha vena ae on the last mirror but one coa- 
en we are sure that the pencil retraces its steps, returns in ie 
to the plane mirror, and all the rays go back primes the mark, point 
for point, as they went forth. 
“This return of the pencil may be proved on an soceaitla imag apa 3 
reflecting the pencil to one side by a airy of glass at an angle of 4 
and examining it through a  & A eas cabal SS besosein 
pages, cles 
lc ated ~ with the mark ee the Erélined ag one solid 
Piece of appara' 
“The real i etn! sent into the microscope, and formed by the returning 
oo partially reflected, occupies a definite position in relation to the 
glass and the mark itself. ‘This is 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 
