MONOCHROMATIC LIGHT OF ANT DESIRED WAVE-LENGTH. 
941 
the whole field of an optical instrument, or any portion of it, with light of any wave¬ 
length or of any definite composition whatsoever. 
The instrument has been admirably constructed by Messrs. Troughton and 
Simms, and the author is particularly indebted to Mr, James Simms for the success 
of the optical portion of the apparatus. The author desires further to express his 
indebtedness to the Research Fund Committee of the Cliemical Society for their grant 
to defray the cost of the instrument. 
