408 Transactions of the Society. 
mirror above described being used to cast a spectrum in the plane 
of the lower iris. 
Method 2. — For low-power work with lenses of small N.A., a 
field illuminated with light of one wave-length of larger diameter 
is required than the above methods will provide. Good results 
can often be obtained in these cases without the condenser or with 
its lower lens only. The spectrum is now formed approximately 
in the upper focal plane of the objective, and a slit-shaped stop 
placed there allows only a restricted part of the spectrum to enter 
the eye-piece. 
Theory shows that the spectrum formed by such a mirror- 
grating is crossed by interference bands. If the grating is mounted 
on one side of a slab of glass and the under surface of the glass is 
silvered, the distance between grating and silvered surface is con- 
siderable, and the bands are so fine that special methods are 
required to show their existence. If, however, it is the upper 
surface of the glass that is silvered, and the grating be mounted 
direct on the silver surface, the path difference between grating 
surface and reflector is small, and the interference bands are coarse, 
in one case being 16 in the first order spectra. This arrangement 
has one very marked disadvantage, that only 16 parts of the 
spectrum can be used for illumination ; it has, however, the com- 
pensatory advantage that the brightness in the centre of the bands 
is twice as much as that formed by the arrangement giving uniform 
spectra. 
