of the Light emitted by Luminous Bodies. 25 



made use of, the wick of which was only drawn out 

 so far as to cause it to emit about -4 part as much Hght 

 as the other Argand's lamp, burning with its greatest 

 brilliancy, which was placed over against it. 



In order that in judging of the equality of the shad- 

 ows, my mind might be totally unbiassed by my ex- 

 pectations, or by any opinions I might previously have 

 formed with respect to the probable issue of the va- 

 rious experiments, keeping my eye constantly fixed 

 upon the field of the photometer, and causing the 

 light whose corresponding shadow was to be brought 

 to be of equal density with the standard to move back- 

 wards and forwards, by means of the winch which I 

 had constantly in my hand, — as soon as the shadows 

 appeared to me to be perfectly equal, I gave notice to 

 an assistant to observe, and silently to write down, the 

 distance of the lamp or candle, so that I did not even 

 know what that distance was till the experiment was 

 ended, and till it was too late to attempt to correct any 

 supposed errors of my eyes by my wishes or by my 

 expectations, had I been weak enough to have had a 

 wish in a matter of this kind. I do not know that 

 any predilection I might have had for any favourite 

 theory would have been able to have operated so 

 strongly upon my mind and upon my senses as to 

 have made black and white appear to me otherwise 

 than as they really were ; but this I know, that I was 

 very glad to find means to avoid being led into temp- 

 tatio7i. 



But to return to the foregoing experiments : the 

 results of them, so far from affording means for ascer- 

 taining the resistance of the air to light, do not even 

 indicate any resistance at all ; on the contrary, it might 



