Reflector or Refractor ?

It was discovered, however, in the latter part of the last century, that some lenses
which had been tarnished by the elements transmitted more light than ones that were newly polished; it was found that this resulted from lessened reflections at the tarnished surfaces. Various processes of producing an artificial tarnish were attempted.

At present, in the most satisfactory method, metallic salts (such as magnesium fluoride) are evaporated in a high vacuum onto the glass. Ideally, the refractive index of an anti-reflection fluoride coating should vary from that of glass at the glass-fluoride surface to that of air at the fluoride-air surface, in which case no reflection would occur. Practically, the index of the coating should be equal to the square root of the index of the glass, and its thickness equal to a quarter of a wave length of yellow-green light. Only the light at opposite ends of the visible spectrum is then reflected, amounting in general to less than one per cent of that of the whole, and is detected by the purplish color given to the reflection.

The different fluorides vary in hardness, magnesium fluoride being perhaps the most durable anti-reflection coating for exposed,

9It is common practice to cement together the components of small achromats
when the adjacent convex and concave surfaces of the crown and flint lenses
have the same radii (see Fig. 14), practically eliminating any reflection there.
But on account of the unequal coefficients of expansion of crown and flint glass,
lenses above 3" in diameter are seldom cemented; thus an additional toll of
light is taken at the two inner surfaces.

surfaces. The cost of application of the coatings is high, but thus improved, the refractor again moves up ahead of the mirror in light-transmitting qualities, a lens of about 51/2-inch aperture being the equal, in this respect, of a 6-inch aluminized mirror.

Reflector or Refractor continued