VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 581 



copes is very different from the magnifying power, and that, in the construction of 

 instruments, these 2 powers ought to be considered separately. In order to conduct 

 our present inquiry properly, it will be necessary to examine the nature of luminous 

 bodies, aud to enter into the method of vision at a distance. Therefore, to prevent 

 the inaccuracy that would unavoidably arise from the use of terms in their common 

 acceptation, I shall have recourse to algebraic symbols, and to such definitions as 

 may be necessary to fix a precise meaning to some expressions which are often used 

 in conversation, without much regard to accuracy. By luminous bodies I mean, 

 in the following pages, to denote such as throw out light, whatever may be the 

 cause of it: even those that are opaque when they are in a situation to reflect .light, 

 should be understood to be included; as objects of vision they must throw out 

 light, and become intitled to be called luminous. However, those that shine by 

 their own light may be called self-luminous, when there is an occasion to dis- 

 tinguish them. The question will arise, whether luminous bodies scatter light in 

 all directions equally; but, till we are more intimately acquainted with the powers 

 which emit and reflect light, we shall probably remain ignorant on this head. I 

 should remark, that what I mean to say, relates only to the physical points into 

 which we may conceive the surfaces of luminous bodies to be divided ; for, when 

 we take any given luminous body in its whole construction, such as the sun or the 

 moon, the question will assume another form, as will appear hereafter. 



That light, flame, and luminous gases are penetrable by the rays of light, we 

 know from experience*; it follows therefore, that every part of the sun's disc 

 cannot appear equally luminous to an observer in a given situation, on account of 

 the unequal depth of its luminous atmosphere in different places -j-. This regards 

 only bodies that are self-luminous. But the greatest inequalities in the brightness 

 of luminous bodies in general, will undoubtedly be owing to their natural texture; 

 which may be extremely various, with regard to their power of throwing out light 

 more or less copiously. Brightness I ascribe to bodies that throw out light; and 

 those that throw out most are the brightest. It will now be necessary to establish 

 certain expressions for brightness in different circumstances. In the first place, 

 let us suppose a luminous surface throwing out light, and let the whole quantity 

 of light thrown out by it be called l. Now, since every part of this surface 

 throws out light, let us suppose it divided into a number of luminous physical 



* In order to put this to a proof, I placed 4 candles behind a screen, at % of an inch distance from 

 each other, so that their flames might range exactly in a line. The first of the candles was placed at 

 the same distance from the screen, and just opposite a narrow slit, \ of an inch long, and \ broad. 

 On the other side of the screen I fixed up a book, at such a distance from the slit that, when the first 

 of the candles was lighted, the letters might not be sufficiently illuminated to become legible. Then, 

 lighting successively the 2d, 3d, and 4th candles, I found the letters gradually more illuminated, so 

 that at last I could read them with great facility ; and by the arrangement of the screen and candles, 

 the light of the 2d, 3d, and 4th, could not reach the book, without penetrating the flames of those 



that were phced before them. f See the paper on the Nature and Construction of the Sun, Phil, 



Trans, for l?yo, page 4b\— Orig, 



