AND INDUSTRY. 
SCIENCE 
it is not possible to measure this focal length with any degree of accuracy 
except in the optical workshop by optical means.) The rays of light 
from the object, on passing through the lens, are refracted or changed 
in direction, and meet at the focus of the lens; beyond which they diverge 
again where they meet the eye. The image is seen in the direction from 
which the rays entering the eye appear to converge. 
This is true with one kind of glass for rays of the same colour only. 
As light consists of rays of many different wave lengths, a ray of white 
light is not only refracted, but each wave length is refracted to a 
different extent, the short waves (e.g., blue) will cross the axis nearer 
to the lens than the longer wave lengths (e.g., red), and there will 
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PORTION OF PROBOSCIS OF BLOWFELY. {See topmost portion of block on other page.) 
Sul further magnifisd—about 620 times. Shows spiral thickening of breathing tubes, 
and th2 minute hairs that cover the whole surface. 
result a superposition of coloured images, none of which are perfectly 
distinct. This defect, namely, that light of different colours is refracted 
to different extents, is called chromatic aberration. The amount to 
which the colours are separated is known as the Dispersion. The power 
of dispersion varies in different kinds of glass according to their com- 
position, and it does not follow that the dispersion produced by two kinds 
will be in the same ratio as the refraction caused by them. Two glasses 
300 
