40(3 
Xix.— MONOCHROMATIC ILLUMINATION. 
By H. Hartridge, M.D., Sc.D., F.R.M.S., Fellow of King’s 
College, Cambridge. 
O ' O 
(Read March 15, 1922.) 
One Text-Figure. 
Of the three methods of obtaining monochromatic illumination, 
viz. (a) filters, ( b ) mercury vapour lamp, and (c) prism, or grating 
spectroscope, the latter has the advantage of providing light of any 
required wave-length. It has the disadvantage, however, of 
requiring special and elaborate apparatus, and this has prevented 
its use becoming more general. It is the purpose of this paper to 
describe how an effective but simple monochromatic illuminating 
apparatus can be constructed. The spectrum-producing part of 
the apparatus consists of an ordinary celluloid replica diffraction 
grating which is mounted over and parallel to an ordinary plain 
silvered mirror similar to, and interchangeable with, the ordinary 
microscope substage mirror. The surface of the grating may be 
protected from injury by a piece of plain glass mounted over the 
top of it. 
A beam of parallel light incident on this forms the usual sets of 
first and second order spectra, not only when the light is first incident 
on the grating, but also after it has suffered reflection and emerges 
again through the grating. This causes the intensity of the spectra 
for unit dispersion to be nearly double those produced when a beam 
passes normally through a replica grating in the usual way. 
Method 1. — For high powers ; the usual substage condenser is 
used, and is focussed in the plane of the specimen. If the working 
distance of the condenser is somewhat short a suitable negative 
spectacle lens of 5 or 6 dioptres focal length may be placed in the 
stop carrier of the condenser. The special grating mirror is now 
attached to the tail piece with the grating rulings horizontal, and 
a beam of parallel light from any suitable source of white light is 
caused to fall on it. By rotating the mirror about the horizontal 
axis either the direct beam of white light, or any part of the first 
order spectrum from red to violet, may be caused to fall on the 
specimen. 
For the spectral light to consist of nearly homogeneous light 
(light restricted to rays of nearly the same wave-length), and at the 
same time to illuminate a considerable width of the specimen, the 
light source should be small in the vertical plane and spread out 
in the horizontal. I have found that a 1 ampere Nernst filament 
