Page 253 - Modern Optical Engineering The Design of Optical Systems

Optical Materials 233

one should consult the manufacturer’s literature for specific details
regarding any given cement.
A rarely used method of fastening optical elements together is by
what is called optical contact. Both pieces must be scrupulously cleaned
(often the final cleaning is with a cloth slightly stained with polishing
rouge) and laid together. If the surface shapes match well enough, as the
air is pressed out from between the pieces a molecular attraction will
cause them to adhere in a surprisingly strong bond, which will withstand
a force of about 95 lb/in . Usually the only way properly contacted sur-
2
faces can be separated is by heating one of them and allowing thermal
expansion to break the contact (it often breaks the glass as well).
Occasionally, soaking in water will separate the pieces.
Optical liquids are used primarily for microscope immersion fluids
and for use in index measurement (in critical-angle refractometers).
For microscopy, water (n d 1.33), cedar oil (n d 1.515), and glycerin
(ultraviolet n 1.45) are frequently utilized. For refractometers
alpha-bromonaphthalene (n 1.66) is the most commonly used liquid.
Methylene iodide (n 1.74) is used for high index measurement (since
the liquid index must be larger than that of the sample to avoid total
internal reflection back into the sample).

Bibliography

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