JOURNAL 
OF THE 
ROYAL MICROSCOPICAL SOCIETY. 
FEBRUARY 1892. 
TRANSACTIONS OF THE SOCIETY. 
I . — Further Notes on the Monochromatic Illuminating Apparatus. 
By E. M. Nelson, F.R.M.S. 
(Bead 18lk November , 1891.) 
Plate I. 
Since my former paper on the subject I have made critical experi- 
ments with this method of illumination, the result being that I am 
so impressed with the advantages gained by the method that for all 
delicate work I intend to use it exclusively. 
The accompanying diagrams illustrate wherein the efficacy of 
monochromatic illumination by prism dispersion excels that by ab- 
sorption media. Fig. 1 shows the spectrum at the back of a 1/2-in. 
apochromatic objective, as seen when the eye-piece is removed, the 
object on the stage being a P. formosurn, and the illumination a 
small cone of monochromatic red light, as in this case it is the 
spectra and not the object we require to examine. 
The figure shows that we have a dioptric beam of monochromatic 
red light surrounded by four half-discs of red spectra of the first order. 
Fig. 2 shows that, with precisely the same conditions, if we sub- 
stitute monochromatic green light for the red we shall bring in the 
whole of the inner four spectra of the first order. 
Fig. 3 shows the same thing with blue light substituted for green. 
We have now nearly half of the outer four spectra of the first order 
brought within the grip of the objective. 
Fig. 4 shows the same object under the same conditions illumi- 
nated by so-called monochromatic blue light, obtained by absorption 
media. The red in the spectrum is still conspicuous. The blue 
edges of the four outlying spectra of the first order are apparently not 
brought so far within the grip of the objective. Theoretically they 
are of course precisely in the same position as in the other case, fig. 3, 
but practically they are not so conspicuous because of the existence of 
other kinds of light. Be that as it may, it stands to reason that 
they can have no possible visual effect in the presence of the much 
more powerful visual exciting rays such as orange and yellow-green. 
[Mr. Haughton Gill has called my attention to the Zeltnow 
1892. " B 
