Dr. Rezner’s Mechanical Finger (Fig. 265) for selecting and arranging diatoms, adaptable to any microscope, is made to slip on to the objective far enough to have a firm bearing, and so that the bristle point can be brought into focus when depressed to its limit. It is clamped in its place by a small thumb-screw. The bristle holder slides into its place, and is carefully adjusted to the centre of the field. When using the finger, the bristle is first raised by means of the micrometer screw till so far within focus as to be nearly or quite invisible, then the objective is focussed on to the slide, and the desired object sought for and brought into the centre of the field; the bristle point is then lowered by the screw until it reaches the object, which usually adheres to it at once, and can then be examined by rotating the bristle wire by means of the milled head. [Illustration: Fig. 265.--Rezner’s Mechanical Finger.] The medium used for mounting diatomaceæ is of considerable importance, inasmuch as their visibility is either diminished or much increased thereby. Professor Abbe, experimenting with the more minute test objects, diatoms, &c., found monobromide of naphthaline gave increased definition to most of them. This liquid is colourless, somewhat of an oleaginous nature, and is soluble in alcohol. Its density is 1·555, and refractive index 1·6. Its index of visibility is about twice that of Canada balsam. Taking the refractive index of air as 1·0, and diatomaceous silex as 1·43, the visibility may be expressed by the _difference_ ·43. The following table may be constructed :-- Refractive indices Visibility of silex (taken approximately). (Refr. index = 1·43). Water = 1·33 10 Canada balsam = 1·54 11 Bisulphide of carbon = 1·68 25 Sol. of sulphur in bisulph. = 1·75 32 " phosphorus " = 2·11 67 These data relating to visibility must be taken in connection with the numerical aperture of the objectives and of the illuminating pencil. The effect produced on diatoms is very remarkable, the markings on their siliceous frustules being visible under much lower powers. So that the visibility of the diatom mounted in phosphorus as compared with balsam is as sixty-seven to eleven; in other words, the image is six times more visible. Mr. Stephenson’s phosphorus medium is composed of a solution of solid or stick phosphorous dissolved in bisulphide of carbon. Great care is required in preparing the solution owing to the very inflammable nature of the materials. So small a quantity of the bisulphide of carbon is required to dissolve the phosphorus that the diatom may be said to be mounted in nearly pure phosphorus. Remarkable enough, this medium has the reverse effect upon such test-objects as podura and lepisma scales. These lose their characteristic markings.