6 



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 Systeme 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 pal- 

 pable 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 men- 

 tion 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. Bouguer, 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 -rVths of the whole light it receives, and that in a te- 

 lescope of his own construction with two lenses this loss amounts to 

 somewhat less than -ro-ths. The Doctor now enters into a full investi- 

 gation 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. 



