1(34 
Transactions of the Society. 
IV. — An Inexpensive Screen for Monochromatic Light. 
By J. William Gifford, F.K.M.S. 
(Rtad 20 th December , 1893.) 
Plate "V. 
Monochromatic light for use with the Microscope may, as far as I 
know, be obtained in three different ways : — 
(1) By the use of a prism. 
(2) By an incandescent gas or vapour. 
(3) By means of a coloured screen. 
The beautiful apparatus used and shown by Mr. Nelson some time 
since is an instance of the first. By this the rays proceeding from 
an incandescent, lime, or other radiant, after passing through a slit, 
are dispersed by a prism of glass, and by means of a second slit any 
portion desired may he selected from the spectrum and used for the 
purpose required. But this apparatus is expensive, and, unless a 
second prism he used with the second slit to recompose it, the light 
thus obtained, although good in practice, does not really produce a 
uniformly monochromatic field of view, that is to say, the illumination 
at any one point is not exactly the same as at any other point, and 
one side of the object in the microscopic field will be illuminated by 
light many wave-lengths behind that by which the other side is 
illuminated. 
As an example of the second method I will refer to the light pro- 
duced by burning metallic sodium or thallium, or a salt of either, in 
the flame of a Bunsen burner, using a suitable support. By this 
method I was enabled, two years ago, to resolve Ampliipleura pellucida 
into distinct dots. But it is difficult to feed the flame continuously, 
so as to make this method available for photography, and the plates 
used have to he bathed in erythrosine or cyanine, or some other dye, 
in order to make them sensitive to these wave-lengths. 
The third method, that of the coloured screen, is not subject, as 
far as I know, to any of these drawbacks, hut, in the form in which it 
has hitherto been mostly used, that of the trough containing chrome- 
copper solution, either the light is insufficiently monochromatic, that 
is to say, the band seen in the spectroscope, when it is so examined, is 
EXPLANATION OF PLATE V. 
Fig. 1. — Solar spectrum without screen. 
n 2. „ with screen, malachite-green in aqueous solution, prolonged 
exposure. 
„ 3. „ „ yellow glass. 
v 4. „ „ malachite-green, glycerine and picric acid. 
„ 5. „ „ effect of 3 followed by 4. 
n 6. — Podura scale x 960, by P. and L. 1/10 in. achromatic N.A. 1*5, with screen 
as in fig 4. 
