EXAMPLES OF PROCEDURE. 
89 
or diminishes in size. If the size increases the light is 
too near the bull’s-eye, if it diminishes and finally 
disappears the light is too far from the lens. Midway 
between these points is the true principal focus of the 
paralleliser, and at that distance it should be fixed. 
The paralleliser must also be centred to the radiant, 
and this position may be found by placing the eye 
centrally in the rays proceeding from the paralleliser. 
Fig. *|8 shows the appearances seen by the eye in this 
position when the paralleliser is properly, and impro¬ 
perly, placed with regard to the radiant, E representing 
the radiant and P the lens; A shows the appearance 
p BCD 
Fig. 28. (Nelson.) 
when the position of the paralleliser is correct, B 
when the lens is too near the light, C when it is too far 
away, D when the light is in the focus of the lens, but 
not central. 
Having now interposed the paralleliser as above 
directed, but not having moved the light, we proceed to 
centre and focus the condenser to the parallel beam, 
the focus being different from that for the diverging 
beam. To effect this, we place in the paralleliser a small 
aperture of the iris, or the opaque disc with the aperture 
described on page 84, and we rack the condenser till 
we get the image of this aperture sharp on the object, 
