Estimation Of Magnifying Power. While the table given on page 899 is of interest and service, yet, owing to slight variations therefrom in almost every microscope, each operator always prefers to test his own by one or all of the following methods:
(a) By taking absolute measurements. Ten inches are recognized generally as the normal length of distinct vision with the naked eye. Now, suppose the distance from an object in focus to the upper end of ocular is 10 inches, and that the 2-inch ocular and 1-inch objective are in service, we will then have the ocular focusing at 2 inches what the eye does in 10, or it magnifies 5 diameters - 10 ٪2 = 5; also the objective focusing at 1 inch what the eye does in 10, or it magnifies 10 diameters - 10 ٪ 1 = 10; consequently these two combined - 5 x 10 = 50, which is the total magnification of the instrument as arranged. If we use 1-inch ocular and 1/5 objective, we have 10٪l = 10 = magnification of ocular alone; 10 ٪ 1/5 = 50 = magnification of objective alone; hence the two combined - 10 x 50 = 500 diameters = combined magnification.
(6) By a stage micrometer and a 2-inch boxwood rule. This micrometer is but a glass slide having 1,000 ruled lines to the inch. When this is focused and the rale placed in front of and parallel with it on the stage, we can compare the two simultaneously by looking at the micrometer through the microscope with one eye and at the ruler with the naked eye. If the micrometer spaces now appear 1/2 inch apart, the magnifying power is 500 diameters; if 1 inch apart, then 1,000 diameters.
(c) By stage micrometer and camera lucida. This gives greater accuracy, and is accomplished by focusing stage micrometer and placing a camera lucida on the eye-piece. To one side in same plane as stage place a sheet of white paper at right angles to the object viewed, and upon this will be projected the image of the lines, which then can easily be drawn and the distance between any two measured. Suppose they are 1/5 inch apart; now those on micrometer are 1/1000 inch apart hence magnifying power is 200 diameters - 1/5 ÷ 1/1000 = 5/1000 = 200. Instead of the camera lucida, an eye-piece micrometer in conjunction with the stage micrometer can also be used with equal if not better results.
Hardening. If tissues to be examined are not sufficiently firm to allow satisfactory cutting - as tender parenchyma of non-vascular plants, they should be hardened by soaking several hours in diluted alcohol, then in pure alcohol. The employment of several alcohols varying in strength prevents tissue-contraction by osmotic action. Alcohol here dissolves resins, volatile oils, chlorophyll, thus acting as a bleaching agent. It coagulates and kills protoplasm without impairing its structure, also renders it more opaque, when it may readily be stained with the various fluids; it also dehydrates tissues previous to being mounted in Canada balsam.