180 Mr. Potter on a new Photometer by Comparison. 



2*25 

 what it is in fact. Taking ju,' = 2'5, we find ^ . , or 18-36 



rays should be reflected of every 100 incident; whilst experi- 

 ment shows it to be only somewhat more than 9*, and perhaps 

 about 9-3. 



I have applied the photometer to a few other substances, 

 and the results of my observations are given in the following 

 Table. 



Substance examined. 



Incidence. 



Mica 



Do 



Do 



Do 



Do 



Do . 



Do.(another piece) 



Do 



Selenite 



Do 



Do . 



Do. (another piece) 



Do 



Iceland spar . . . 



Do 



Do 



Do . 



Do.(another piece) 



Do 



Rock crystal . . . 



Do 



Do 



Do . 



Do. (another piece) 



Do 



Amethyst . . . 



Do 



Emerald .... 



Do 



Do 



Do 



5° 

 5 



20 



20 



30 



30 

 5 

 5 

 3 

 5 



45 

 5 



20 

 5 

 B 

 5 

 5 

 5 

 5 

 

 

 

 

 5 



lO 

 5 

 5 



10 



10 



30 



30 



Corresponding 

 Incidence on 

 Crovvn-e;lass. 



20° 0' 



19 



25 30 



25 



34. 



35 



23 



19 

 3 



10 



45 



5 



20 



19 

 18 



15 

 17 

 22 

 22 

 14 

 14 

 12 30 



16 

 12 

 14 



20 

 22 

 22 



24 

 40 

 40 



Reflected of 



every 100 



Rays incident. 



3-83 

 3-80 

 4-01 

 3-99 

 4-38 

 4-43 

 3-92 

 3-80 

 3-49 

 3-60 

 5-20 

 3-52 

 3-83 

 3-80 

 3-78 

 3-78 

 3-75 

 3-89 

 3-89 

 3-68 

 3-68 

 3-65 

 3-73 

 3-64 

 3-68 

 3-83 

 3-89 

 3-89 

 3-95 

 4-76 

 4-76 



The natural surfaces, and as recent as possible for mica, 

 selenite, and Iceland spar, were used in all the above, except- 



