ILLUMINATION. 
83 
two gases gradually, first a little H., then a little O., 
then more H. and more O., till there is at last so much 
H. that the O. cannot whiten it, and then to turn down 
the H. till the light is small in area and pure in quality. 
A red light betokens too much H. ; a lurid light means 
too much O. for the H. 
The jet should be supported in such a way that it 
can be moved both vertically and horizontally, and be 
firmly clamped when the central position is found, as 
will be described later. 
What is known as the “ bull’s-eye ” may very well be 
considered as part of the illuminating system, but we 
prefer to call it the “ paralleliser,” as that represents its 
true function in most cases. It is in its simplest form 
a plano-convex single lens, with a diameter of about 
three inches. Being placed between the light and 
the condenser, the light being in its principal focus, 
it casts a parallel bundle of rays on the condenser, 
which has the effect of spreading the light evenly over 
the field on the ground-glass in cases where, without 
the paralleliser, we should have the field unevenly 
lighted ; but on account of not being corrected in any 
way, it introduces certain errors into the projected 
image. Much to be preferred is a “doublet” paral¬ 
leliser, somewhat similar to a small “ Herschel ” con¬ 
denser ; and fig. 27 shows a very good form designed by 
Mr. E. M. Nelson and used by many workers, ourselves 
among the number. 
This instrument can also be used as a condenser, 
i.e., to cause the rays to converge on an object on the 
