Mr. Potter's Reply to Mr. Wheeler. 485 



only the image formed by it, according to the ordinary rules 

 <)t" catoptrics for plane reflectors; therefore it is immaterial at 

 what angles of incidence the light be reflected, provided that 

 the same proportion of the incident light be reflected by both; 

 and conversely this must be clearly the case, when equally 

 bright images are formed of equally bright objects. 



To find the reflective power of diamond at various angles 

 of incidence, when that of glass is known, we ascertain the 

 angles of incidence in the comparative photometer at which 

 the images appear equally bright. Thus suppose that for any 

 given angle of incidence /, on diamond, the image is equally 

 bright with that formed by glass at an angle of incidence ic^ 9 

 then thepartsof the pasteboard themselves being equally bright 

 also, we have clearly as many rays reflected by diamond at 

 an angle of incidence i^ as by glass at an angle 4 . But since 

 we can obtain large and plane surfaces of glass, we can deter- 

 mine its reflective power for all incidences by independent 

 methods, and therefore when this is known we can by the 

 comparative photometer deduce that of diamond from it, 

 which cannot be obtained in surfaces sufficiently large for 

 those methods. Hence the value of this method of photo- 

 metry by comparison. 



The comparative photometer is also serviceable in other 

 cases, as for instance in determining the relative intensity of 

 the light in Newton's rings seen by transmission, but in all 

 cases the principle on which it acts is the same. 



The above is the method I have pursued in the investiga- 

 tion to which Mr. Wheeler objects, and there is clearly no 

 other formula involved in the problem than that by which 

 we calculate the reflection by glass at any given angle of in- 

 cidence. 



The magnitude of the reflecting surfaces cannot in the least 

 affect the brightness of the images formed by them ; but the 

 eye may nevertheless err in comparing illuminated surfaces 

 of different magnitudes, as for instance a square inch of white 

 paper laid on a dark substance may appear brighter than a 

 square foot of white cloth similarly placed, on account of the 

 contiguity of the dark substance, or the prejudice of the eye, 

 although equal portions might appear equally bright; hence 

 as a precaution of experiment, it is advisable, if not necessary, 

 to compare areas of the pasteboard surface which are nearly 

 equal in magnitude, and the reflector should therefore be 

 so, or if one is much more inclined to the visual ray it ought 

 to be larger proportionally. 



I have been more particular on this point from its having 

 apparently been in some measure the cause of Mr. Wheeler's 



