Diacthlic. A term applied to any medium through which the actinic rays of light can pass. Substances which allow only non-actinic rays to pass are termed adiactinic.
Dialyser. This is sometimes used in the washing of emul-sions, and can be prepared as follows: - Take a hoop of gutta percha, and over it stretch a piece of parchment paper, this being tied on tightly. It is used by floating it in a vessel of dis-tilled water, and the materials to be dialysed are placed in the hoop. All bodies which will crystallise will pass through the septum of parchment paper, leaving those which will not crystal-lise in the dialyser. In the case of emulsions the unnecessary salts, such as nitrate of potash, etc., pass through the septum, leaving the colloid gelatine holding the sensitive silver salt.
Diameter. Any straight line passing through the centre of a circle and touching the circumference at opposite points is thus termed. If the diameter of a circle is known, multiplying that by 3-14159 will give the circumference, and vice versa; and the diameter squared and multiplied by 7854 will give the area of the circle, and the cube of the diameter multiplied by '5236 will give the solid contents of a sphere.
Diamidophenol. See Amidol.
Diamond- A hard crystalline form of carbon. Its high refractive power, 2.47 to 275, together with its comparatively low dispersive power, would render it valuable for use in the con-struction of lenses, were large clear stones available and easily workable. Andrew Pritchard, working early in this century, did actually construct lenses of diamond. The natural edge of a small crystal is the effective agent in the glass-cutter's diamond, in the use of which the most important matter is to maintain the mounting of the instrument at the same angle with the surface of the glass when once the best angle for cutting is found. In cutting dry plates or negatives, it is convenient to cut on the film side if the diamond works satisfactorily in this way, otherwise the cut may be on the glass side. When the glass has separated, and the film is like a hinge, a sharp folding of the plate in the reverse way will generally disrupt the film without causing any stripping.