1 6 Experiments on the Relative Intensities 



Having now, as I imagine, sufficiently described all 

 the essential parts of these instruments, it remains for 

 me to give some account of the precautions which, 

 from experience, I have found it necessary to employ 

 in making use of them. 



And, first, with respect to the distance at which 

 lights whose intensities are to b.e compared should be 

 placed from the field of the photometer, I have found 

 that, when the weakest of the lights in question is about 

 as strong as a common wax candle, that light may 

 most advantageously be placed from 30 to 36 inches 

 from the centre of the field ; and when it is weaker or 

 stronger, proportionally nearer or farther off. When 

 the lights are too near, the shadows will not be well 

 defined ; and when they are too far off, they will be 

 too weak. 



It will greatly facilitate the calculations necessary in 

 drawing conclusions from experiments of this kind, if 

 some steady light, of a proper degree of strength for 

 that purpose, be assumed as a standard by which all 

 others may be compared. I have chosen for that pur- 

 pose an Argand's lamp, made in London, and very 

 well finished ; and though the quantity of light emitted 

 by this or any other kind of lamp is very various, 

 depending in a great measure upon the length to 

 which the wick is drawn out, yet I have found by 

 repeated trials that this lamp, once properly adjusted, 

 continues to emit light more equally for a considerable 

 time than any other lamp, and much more so than any 

 candle whatever. 



At the beginning of each experiment I adjust this 

 standard light in the following manner: Having 

 placed the lamp upon its carriage, at the distance of 



