The refractive indices of all glasses at present available lie between 1.46 and 1 90, whereas transparent minerals are known having refractive indices lying considerably outside these limits; at least one of these, fluorite (calcium fluoride), is actually used by opticians in the construction of certain lenses, so that probably progress is to be looked for in a considerable widening of the limits of available optical materials; possibly such progress may lie in the direction of the artificial production of large mineral crystals.
Early opticians and microscopists gave their chief attention to the improvement of the simple microscope, the principle of which we now explain.
Compound Microscope The view held by early opticians, that a compound microscope could never produce such good images as an instrument of the simple type, has proved erroneous; and the principal attention of modern opticians has been directed to the compound instrument.
The development of this instrument was studied by opticians for the remainder of the first half of the 19th century; the last improvement apparently was made by P. G.
Opticians should supply sufficient information of the dispersive properties of their materials to allow dµ/dX to be calculated easily for different parts of the spectrum.