310 Professor Piazzi Smyth on a Centauri, 
between the heliotrope and observer. At the distances stated, 
the light was just visible to the naked eye in clear weather, 
and when seen over a valley ; if the ray grazed near the sur- 
face, its light was much reduced. On one occasion, I em- 
ployed a heliotrope at 6} miles, and used an aperture of 
2 inch, and found it rather brighter than my usual allowance, 
so that probably 6} or 7 miles would be the nominal dis- 
tance for that size. This agrees well enough with the rest 
of the scale, but there is no need to employ a conjectural 
quantity ; and if the rate of absorption be computed corre- 
sponding to the above, a very close agreement will be found ; 
and the mean of the whole shews a loss of ‘0610 in passing 
through one mile of atmosphere, with the barometer at 27:0 
inches, that being about the average height at my stations. 
With barometer = 30°0 inches, the value will be (0672, and 
the loss in passing from the zenith through a homogeneous 
atmosphere of 5:2 miles will be *303, or only about one per 
cent. less than Professor J. D. Forbes’ result (Phil. Trans., 
1842, Part 2) ; and as my air was considerably drier than his 
(mean humidity probably not much above ‘30 instead of -56), 
this will probably account for the difference. 
Applying this, then, to the intensity of solar light, and eli- 
minating the atmospheric absorption, the diameter of the ob- 
ject that shall be about as bright as Venus, comes out 0-0205 
inches per mile, or in are 00668. Now, the object viewed 
being the reflection of the Sun from a metallic surface after 
passing twice through glass, by which at least § would be lost; 
the diameter of the portion of the Sun which would be equally 
bright, is therefore 0:045”. Also, we never observed between 
21" and 3", consequently the Sun’s altitude was always below 
45°, at a mean it might be taken about 23°, where the loss of 
light is ‘41 per cent. more than in the zenith ; consequently 
there remains ‘59, /59 = -77 and77 x -45 = -35; therefore 
the portion of the zenith Sun equal to Venus is 0-035’. Now, 
I do not know the ratio of Venus’ light to Sirius, but shall not 
be far wrong in estimating it at 4-0 ; then the portion of Sun 
equal to Sirius will be 0-017’, and as Sirius is 4 x a Centauri, 
this last will be = 0-009” of the Sun, or the Sun must be re- 
moved to 213,333 times its present distance to = a Centauri, 
or must have a parallax = 0-97”.. This makes a Centauri, in 
