36 BULLETIN 448, U. S. DEPARTMENT OF AGRICULTURE. 
make dyeings to any convenient standard depth, and descriptions can 
only indicate in a general way what may be expected. Not only will 
the appearance of the dyed wool under the influence of different 
reagents vary somewhat with the concentration of the dye present, 
but the shade of the dry fiber also may vary with the concentration of 
the dye. For example, dyeings from some of the oranges are almost 
yellow when only a little color is present, but are a much redder shade 
when more dye is used. 
Color changes similar to those taking place on dyed fibers are pro- 
duced in most cases by the given reagents in solutions of the dyes, and 
the conditions are under much more exact control. So in some cases 
it is advantageous to compare the solution of the dye under examina- 
tion with solutions of known colors, all being brought as nearly as 
possible to the same dye concentration, and to the same acid or alkali 
normality. An exact statement of shade can be given best by spectro- 
photometric data (according to Vierort's method) under prescribed 
conditions of temperature and concentration. This is desirable in 
some cases, as when dealing with mixtures of Ponceau 2 E, and Pon- 
ceau 3 It. For the somewhat related and more rapid spectroscopic 
method of measuring the spectral position of maximum light absorp- 
tion in dilute solutions, the treatises of Foraranek, of Formanek and 
Grandmougin, and of Mulliken may be consulted. However, the 
"spot reactions/' if not of the greatest exactness, are sufficiently exact 
for most purposes, and are especially convenient in inspection work, 
where it is well to keep a specimen of the color dyed on cloth. 
