doubt the correctness of the Copernican theory. But it recurred to me that Venus is accredited with having a highly refractive atmosphere, through which those rays of sunlight must pass which outline her dark disc on the face of the sun during a transit; and might not this condition cause complications which have not as yet been properly estimated? Let us as briefly as may be, examine this question:

In Fig. 4, (next page,) E represents the earth, S the sun, and the small dark disc c, Venus in central transit. By the aid of this diagram we will now consider the capacity of Venus' atmosphere for refracting rays of sunlight passing through it.

Those rays passing through its outer and thinner portions would naturally be refracted least, while refraction would gradually increase centreward, or toward the planet.

The same effect would thus be produced as if the rays were passing through a lens having a cross section of the peculiar form represented by a a, in Fig. 4, which is there substituted for the planet's atmosphere.

Then, referring to the diagram, it is evident that the ray m, in passing from sun to earth will suffer little or no refraction from the lens, since it passes through its nearly plane outer edge; but the ray n, passing as shown, through a highly refractive part of the lens, will be turned from its course and brought to a focus with the ray m, at the earth's surface.

Observers at this right hand earth station would then see the sun in the direction of the angle of vision described by m and n as they approach the earth, and slightly enlarged, as it really appears during a transit of Venus, while the planet would appear on its disc, as at b, instead of at b', where it would appear, relatively to the sun, if it were visible and there were no refraction; while observers at the opposite station, as shown by the dotted lines, would witness the same result in reversed order, thereby greatly reducing the true parallax. This might also explain the cause of that drawn out internal contact called "the black drop" observed during a Transit of Venus.

But it will doubtless be objected that the atmosphere of Venus has no such extent as this diagram would appear to give it; but as is evidenced by the narrow, bright belt which is seen to encircle the planet when entering on a transit, the hight of its atmosphere must be many times less than its own diameter. I would answer by suggesting that this luminous, or visible belt is only the misty, or vapor-charged portion of the planet's atmosphere, while the invisible, or clear air beyond it, thins out gradually into the æther of space.

Let us again, by the aid of Fig. 5, (page 200) consider what obstacle the atmosphere of our globe interposes to gaining a true parallax of a heavenly body:

In this diagram, E represents a hemisphere of the earth, with its atmosphere a a, while m and represent stars sending rays of light to the earth. Now, if we consider this atmosphere, a a, as a lens of an equal density throughout, a little knowledge of the science of optics will tell us that these rays could enter the lens at points where

they might be refracted parallel to, though a little removed from each other, as shown at the left of the figure; but if the two rays entered the lens a little closer together, so that they came to a focus at the earth, then they would no longer be refracted parallel, and we would get a parallax.

But as our atmosphere is not of uniform density, and does not refract rays of light thus, but in curved lines, as shown at the right of the figure, is it not equally clear that these rays could be brought together on the same plane at the earth's surface by atmospheric refraction, as is there shown? And as we behold objects in the direction of those portions of rays reaching our eyes, no parallax would be obtained; and I believe this to constitute the chief bar to gaining a parallax of a heavenly body by observing it from opposite sides of the earth.

And would not this also assist Venus' atmosphere in still further reducing the sun's parallax toward that narrow limit observed during a transit of Venus, aside from other possible causes, whose existence and effects