3B Scientific Light Box - Manual (Page 20)

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rays, which affect the eyes the most, come in between. A screen placed so as to catch the yellow focus
would show an image with a purple border, the edges being bluish.
The correction of this defect is accomplished by combinations of lenses of different sorts of glass. Flint
glass, for instance, spreads the colors apart more widely than does the crown glass. A diverging lens of
flint glass placed close to a converging lens can bring the spread colored rays back to parallelism.