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Slide Diatoms
The Refractive Index of Media by David Bunch
Toluene solutions dry more rapidly than those in xylene, some technicians think, too rapidly. Clarite may be secured as a lump resin if preferred, when it is made up as a 60% solution; 60% resin and 40% xylene, toluene, benzene, or dioxan, the latter preferred for the dioxan technique. A second Neville product is termed Clarite X, which is somewhat cheaper, yellows slightly but not as much as the natural balsams, and has a higher refractive index. Isobutyl methacrylate is a duPont preparation that gives a water-white mountant hardening in five to ten minutes. O'Brien and Hance suggested that this reagent can take the place of both mountant and cover glass: if a stained and cleared section is dipped into a thin solution of this substance and withdrawn with care so that the liquid drains off evenly from the slide, the thin film remaining will harden rapidly and form an excellent protection for the material. It can be polished with a paper handkerchief and can be written on with ordinary ink, doing away with the paper label. Or, a label may be prepared in the ordinary manner and pasted to the slide, then coated with methacrylate, thus giving the label a transparent protective coating. The back of the slide maybe left with a film of the solution, may be cleaned with a cloth moistened in benzene or xylene, the solvents for this resin.
A more technical point, not heretofore discussed, concerns the refractive index of these media, a very important consideration for many cases. Reduced to simplest terms, refraction is the bending of a ray of light when passing obliquely from a medium of one density into another medium of a different density, as when a ray passes from water into air, or from a piece of tissue into balsam. Refractive index (R.I.) compares the amount of bending in any given substance with that in a vacuum, arbitrarily designated as 1. Since all materials are denser than a vacuum, all R.I.'s are greater than 1. The figure given for any transparent material indicates the relative bending power of the medium on light rays. Manufacturers use these indices in selecting suitable glass for making lenses, and the microscopist chooses his mountant by the same principle. The rule is that the nearer the R. I. of the medium to that of the object, the less the contrast between object and mountant, and the greater the transparency.
This closeness of R.I. may be desirable in some cases, but if the indices should be identical, the object would become invisible. Conversely, the farther the R.I. of the medium from that of the object, preferably higher than lower, the greater the contrast and less the transparency. Diatoms furnish a good example.
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