Neiv Way of producing Colour-Contrast. By J. Rheinberg. 377 
illuminated portion (plate IX. fig. 5 and explanatory note), we obtain 
a similar effect as with ordinary dark-ground illumination. When the 
object is put into position, the remaining of the area, as may be 
seen by looking down the tube with eye-piece removed, are more or less 
filled with light (plate IX. fig. 6 and explanatory note). When we 
look at the object it appears on a dark ground. The image, however, 
although we will assume we are using an objective absolutely corrected 
for chromatic and spherical aberration, suffers from two defects, the 
one of greater the other of less importance, to understand which we 
must again have recourse to the diffraction theory. We will take 
the case of a normal object, viz. one which has structure of varying 
degrees of fineness to be made visible, and also refracts light to a 
different extent in its various parts. For simplicity’s sake, moreover, 
we will suppose the object to give rise to diffraction-fans having a 
number of regular maxima and minima of light.* Let us divide the 
light incident upon the object into two divisions according to the 
extent to which it may suffer refraction (plate IX. fig. 2). In 
division I. we will place those pencils which are but slightly refracted 
by the object, and the dioptric pencils of which consequently fall 
within the central zone of the objective which is covered with our 
black stop. In division II. we place those pencils which are more 
strongly refracted by the object whose emergent dioptric pencils are 
refracted outside of the central zone. 
(As there appears to have been some doubt upon the meaning to 
be attached to the term “ dioptric ” pencil or ray, it will be as well 
here to state that I use it solely in the sense of the central or axial 
ray of any complete diffraction-fan, viz. that ray which follows the 
ordinary laws of geometrical optics.) 
Having divided the pencils of light according to the extent to 
which they are refracted , we must now subdivide our two divisions 
each into three classes, according to the degree in which the light is 
diffracted, which corresponds to the fineness of the structure. In the 
first class, A, let us place the pencils of light traversing coarse struc- 
ture, the whole of whose diffraction-fan is admitted together to the 
objective, without appreciable angular extension. In the second 
class, B, all finer structure giving rise to a more extended diffraction- 
fan, but such that at least the diffraction-spectra of first and second 
order are admitted by the objective. In the third class, C, we place 
still finer structure from which only diffraction-spectra of the first 
order are admitted. 
Now we are in a position to consider the action of our black 
central stop above the objective. 
Firstly, in relation to division I., which comprises the slightly 
refracted beams, class A, where the total diffraction-fan is admitted 
together to the objective : these are obliterated by the stop — in other 
* The same general principles and reasoning can easily be applied to other 
cases. 
