£o Dr. Herschel on the Power of 



be called self-luminous, when there is an occasion to distinguish 

 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 to the 

 rays of light, we know from experience;* it follows therefore, 

 that every part of the sun's disk cannot appear equally lumi- 

 nous to an observer in a given situation, on account of the 

 unequal depth of its luminous atmosphere in different places.-f- 

 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 ; 



* In order to put this to a proof, I placed four candles behind a screen, at 1 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 

 tip 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 second, third, and fourth candles, I found the letters gradually more 

 illuminated, so that at last I could read them with great facility; and, by the arrange- 

 ment of the screen and candles, the light of the second, third, and fourth, could not 

 reach the book, without penetrating the flames of those that were placed before them. 



f See the Paper on the Nature and Construction of the Sun. Phil. Trans, for 

 *795>page46. 



