SMALL SLIDE RULES WITH MAGNIFYING CURSORS.—Several makers now supply 5 in. rules having the full graduations of a 10 in. rule, and fitted with a magnifying cursor (Fig. 34). This forms a compact instrument for the pocket, but owing to the closeness of the graduations it is not usually possible to make a setting of the slide without using the cursor. This, of course, involves more movements than with the ordinary instrument. It is also very necessary to use the magnifying cursor in a _direct_ light, if accurate readings are to be obtained. If these slight inconveniences are to be tolerated, the principle could be extended, a 10 in. rule being marked as fully as a 20 in., and fitted with a magnifying cursor. The author has endeavoured, but without success, to induce makers to introduce such a rule. The magnifying cursor, supplied by Messrs. A. G. Thornton, Limited, has a lens which fills the entire cursor. It has a powerful magnifying effect, and the change from the natural to the magnified reading is less abrupt than with the semicircular lens. [Illustration: FIG. 34.] THE CHEMIST’S SLIDE RULE.—A slide rule, specially adapted for chemical calculations, has been introduced recently by Mr. A. Nestler. In this instrument the C and D scales are as usually arranged; but, in place of the A and B scales, there are a number of gauge points or marks denoting the atomic and molecular weights of the most important elements and combinations. The scales on the back of the slide are similarly arranged, so that by reversing the slide the operations can be extended very considerably. The rule finds its chief use in the calculation of analyses. Thus, to find the percentage of chlorine if _s_ grammes of a substance have been used and the precipitate of Ag.Cl. weighs _a_ grammes, we have the equation, _x_ = (Cl.)/(Ag.Cl.) × (_a_)/(_s_). Hence, the mark Ag.Cl. on the upper scale of the slide is set to the mark Cl. on the upper scale of the rule, when under _a_ on the C scale is found the quantity of chlorine on D. By setting the cursor to this value and bringing _s_ on C to the cursor, the percentage required can be read on C over 10 on D. The rule is also adapted to the solution of various other chemical and electro-chemical calculations.