1868. Luminous Intensity of Light. 365 
for instance, one is bright green and the other scarlet—there igs 
some difficulty in estimating the exact point of neutrality ; but this 
only diminishes the accuracy of the comparison, and does not 
render it impossible, as it would be according to other systems. 
No attempt has been made in these experiments to ascertain 
the exact value of the standard spirit-flame in terms of the parlia- 
mentary sperm candle. Difficulty was experienced in getting two 
lots of candles yielding light of equal intensities, and when their 
flames were compared between themselves and with the spirit-flame, 
variations of as much as 10 per cent. were sometimés observed in 
the light they gave. Two standard spirit-flames, on the other hand, 
seldom showed a variation of 1 per cent., and had they been more 
carefully made they would not have varied 0-1 per cent. 
This plan of photometry is capable of far more accuracy than 
the present instrument will give. It can scarcely be expected that 
the first instrument of the kind, roughly made by an amateur 
workman, should possess equal sensitiveness with one in which all 
the parts have been skilfully made with special adaptation to the 
end im view. 
