Monochromatic Illumination. 
407 
is admirable, if a black sheet iron disc be mounted behind it, and 
a similar sheet of iron with a slit-shaped aperture be mounted in 
front of it so as to eliminate stray or reflected light. A small 
carbon arc or a Pointolite lamp may be used if efficiently screened. 
In the case of the Nernst lamp an ordinary plano-convex spectacle 
lens of about 8 dioptres focal length is admirable for parallel- 
ising the light incident on the mirror. If the Pointolite or carbon 
arc is used a cylindrical axis vertical of 3 dioptres focal length 
will be required as well. Further details can be gathered from the 
diagram below. 
The calibration of the apparatus in wave-lengths, which is only 
required under special circumstances, is, in my opinion, best effected 
by an ordinary micro-spectroscope eye-piece,* the wave-length of 
the monochromatic light being read off directly on its wave-length 
C, condenser ; MG, mirror with grating ; CL, collimating lens ; 
S, source of light. 
scale. If such an eye- piece is not available an ordinary direct 
vision spectroscope with wave-length scale can be used, but it 
seems that there should be no difficulty in fitting an index arm 
and scale on to the mirror, and then calibrating it with a standard 
monochromatic illuminator or with line spectra of suitable chemical 
elements in the manner employed in the case of an ordinary 
spectroscope. If the apparatus were manufactured this latter 
method would be the best one. 
When the above arrangement is used for high powers, the field 
being small is illuminated by practically the same wave-length 
light all over ; particularly is this the case when the condenser is 
of relatively long focal length. If for special purposes it is required 
to be able to restrict the illuminated field to an area as small or 
smaller than that of the eye-piece, then the special method of 
illumination previously described should be employed,! the grating 
* See Spitta, Microscopy, edition I., p. 426. 
f Journ. Quekett Micro. Club, Series II, No. 85, 1919. 
2 E 2 
