308 
POPULAR SCIENCE REVIEW. 
In a reply to these remarks, Mr. Christie, chief assistant at Greenwich, 
makes the somewhat startling assertion that the diameter of Venus is as 
much diminished by photographic irradiation as the diameter of the sun is 
increased, and that therefore the sum of the diameters is not affected at 
all. But it cannot be admitted that the diameters of Venus and the sun 
are affected to the same degree by photographic irradiation. The blackness 
of Venus and of the sky round the sun may be equal, but the brightness of 
the sun near the limb is very inferior to the brightness near the centre. 
This familiar fact somewhat weakens Mr. Christie’s argument. But, to say 
truth, no one who has studied a photographic picture of the sun can com- 
pare the definition of the limb even with the definition of a sun-spot, far 
less with that of a planet in transit. The appearance of the projected imago 
of the transit of Mercury, in November 1861 and 1869, corresponded precisely 
with the appearance depicted in photographs. 
Dr. Butherfurd, in Part I. of u Papers relating to the Transit of Venus 
in 1874,” * says : u The photograph of the sun will have a greater or less 
diameter by many seconds of arc , according to the energy of the rays which, 
have produced the image ; and this discrepancy may be produced by a change 
in the aperture, in the length of exposure, in the transparency of the atmo- 
sphere, in the hour of the day, or in the sensibility of the chemicals.” To 
assume that the diameter of Venus will be affected in precisely the same 
degree, notwithstanding the marked difference in the conditions, seems 
somewhat unsafe. In the same series of papers all the points recently raised 
in defence *of the photoheliographic method and against the long-focal 
method have been fully considered. 
Bright Arc seen round dark Limb of Venus in Transit. — Mr. Bussell, 
Government Astronomer at Sydney, has obtained photographs showing the 
bright arc seen around the dark part of the disc of Venus between the 
epochs of external and internal contact. At the meeting of the Astronomical 
Society in May, after describing this appearance, he expressed doubts 
whether the bright arc is really due to the refractive action of Venus’s 
atmosphere, because that action would disperse the refracted light between 
the earth and the sun. This is true of the totality of light so refracted, but 
it does not affect any small pencil of light such as the observer’s eye or 
telescope could receive. In fact, the same reasoning would show that the 
sun, as seen by us after geometrical sunset (that is, when a straight line to 
the sun’s upper limb is truly horizontal), ought not to be seen, because the 
air is refracting the solar rays divergingly. 
Diameter of Venus, from recent Transit Observations. — Colonel Tennant, 
from an examination of the recent transit observations, gets the following 
result as to the dimensions of Venus : — 
Mean semi-diameter at mean distance . 8 -"4518 + CK'0008 
The following determinations had previously been obtained : — 
Encke, from transits of Venus 8-303 
Airy, measures on meridian ..... 8-283 
Main, micrometer (double image) .... 8-775 
Stone, measures on meridian 8-472 
Plummer, micrometer (double image) . . . 8-661 
The value used in the American Nautical Almanac . 8-546 
Published at Washington in 1872. 
