6*0 Dr. Herschel on the Power of 



By directing our view to them, and thus penetrating one step 

 farther into space, these stars of the second magnitude furnish us 

 with an experiment that shews what phenomena will take place, 

 when we receive the illumination cf two very remote objects, 

 equally bright in themselves, whereof one is at double the dis- 

 tance of the other. The expression for the brightness of such 

 objects, at all distances, and with any aperture of the iris, 



according to our foregoing notation, will be ~^r', and a 'method 



of reducing this to an experimental investigation will be as 

 follows. 



Let us admit that a. Cygni, (3 Tauri, and others, are stars of 

 the second magnitude, such as are here to be considered. We 

 know, that in looking at them and the former, the aperture of 

 the iris will probably undergo no change ; since the difference 

 in brightness, between Sirius, Arcturus, a Cygni, and (3 Tauri, 

 does not seem to affect the eye so as to require any alte- 

 ration in the dimensions of the iris ; a, therefore becomes a 

 given quantity, and may be left out. Admitting also, that the 

 latter of these stars are probably at double the distance of the 

 former, we have D 1 in one case four times that of the other; 

 and the two expressions for the brightness of the stars, will be / 

 for those of the first magnitude, and -i/ for those of the second. 



The quantities being thus prepared, what I mean to suggest 

 by an experiment is, that since sensations, by their nature, will 

 not admit of being halved or quartered, we come thus to know 

 by inspection what phenomenon will be produced by the fourth 

 part of the light of a star of the first magnitude^ In this sense, 

 I think we must take it for granted, that a certain idea of bright- 

 ness, attached to the stars which are generally denominated to 



