brightness is the effect of a considerable depth of luminous matter, he shows that these differences noways affect the present inquiry: since in all these several bodies, it is the quantity of light emitted by their surfaces which becomes the object of our perception. As the same body, however, may be differently luminous in different parts of its surface, he exhibits a formula by which the aggregate brightness of a body may be estimated. And he closes this part with an examination of the opinion maintained by Lambert in his Systéme du Monde, where he says that an object is equally bright at all distances, and that the sun at the distance of Saturn, or still further from us, would be as bright as it is in its present situation. This assertion taken in the general sense in which it is here expressed, he proves to be a palpabIe contradiction; and only admits it in as far as the celebrated author may mean the intrinsic brightness of a body, which applies to its surface diminished by distance, and not the absolute brightness of the whole.
In the next section the author endeavours to ascertain the general extent of vision with the naked eye. As to those bodies which shine with a reflected light, he asserts that none have yet been seen more distant than the Georgian planet: admitting this as the maximum, it must after all excite our admiration that borrowed light should be perceptible to our naked eye at the distance of no less than eighteen hundred millions of miles; especially if we consider that the light of the sun on that planet is above 368 times less intense than it is on our earth, and that probably two thirds of that diminished light is absorbed by the planet.
The range of natural vision, with respect to self-luminous objects, is incomparably more extended. Passing over the intermediate steps by which our author arrives at his conclusions, we shall simply mention his inference that no single star above nine or at most ten times more distant than Sirius can possibly be perceived by the naked eye; admitting, however, that an accumulation of stars will be perceptible at a far greater distance.
From the power of penetrating into space by naked vision, our author proceeds to that of telescopes. Here he first calculates, by a method recommended by Mr. Bong-oer, the quantity of light absorbed and dissipated by the reflection of the mirror, and refraction of the eye-glasses; and he finds that a common Newtonian with three lenses loses about ggths of the whole light it receives, and that in a telescope of his own construction with two lenses this loss amounts to somewhat less than fiths. The Doctor now enters into a full investigation of the penetrating power of his several telescopes under all the various circumstances he could devise, and illustrates the whole by a great number of observations, which serve to confirm the inferences deduced by him. Here we learn that the penetrating power of his 20-feet reflector, applied to a single star, may extend as far as 612 times the distance of Sirius, and also that his large telescope, with a penetrating power of 192, will show a single star of the 1342nd magnitude.
