tube to be lowered or raised. The fine adjustment consists of a micrometer screw situated at the top of the pillar.

The tube frequently has a draw tube, into the upper end of which the ocular or eye-piece is set. To the lower end of the tube

the objectives are attached. In most modern instruments there is added an especial apparatus called the nose-piece, to which two

or more objectives may be attached, any one of which may thus be brought into position.

Below the stage there may be a sub-stage, which may be provided with a condenser, the Abbé being the best. From the lower portion of the stage near the pillar the mirror bar descends, carry

ing the mirror, and in some instances a diaphragm. In some instruments the diaphragm is situated just below the aperture, and may be of several types; the most useful one is the iris diaphragm, as displayed in the cut. (Fig. 430.)

In the best of modern instruments the whole sub-stage arrangements are bound together in one mechanism, as in the following illustration. (Fig. 431.)

THE OPTICAL PARTS.

The ocular or eye-piece is set into the upper portion of the tube. There are two types of eye-pieces, the negative, or Huyghenian, which is the more commonly used, and the positive, or Ramsden, now rarely seen in microscopic use. In the more advanced types of instruments there is the so-called compensating ocular, which is, however, only used with the newer apochromatic objectives.

The action of the eye-piece is that of a simple magnifier, but it magnifies the real image which the objective produces, as if that image were the original object.

Oculars are designated as high and low, and are measured in inches or millimeters; the longer the ocular, the lower or weaker it is in magnifying power, and vice-versa. (Figs. 432, 433.)

The objective is often called the most important part of the instrument, which is, in many senses, true, for the great advances that have come about in the biological sciences have been mainly due to the advances that have been made in the construction of the objectives.

The objective is placed at the lower end of the tube, either fitting directly into the tube, or into the nose-piece by means of what is known as the Society Screw, which is an internationally adopted size, so that any objective can be used in any stand.

The objective is made up of a series of lenses varying from two to four, and works in a combination as a simple magnifier; it forms a real inverted image in the tube, which is further magnified by the ocular.

Objectives are designated either arbitrarily, as a, b, c, d, e, etc., or according to a system of measurements expressed in fractions in the English or in the metric system. These measurements signify that the magnifying power of any lens, say a inch objective, is equal to the magnifying power of a simple lens whose

FIG. 434.

Construction of Objective.

focal distance equals inch. In general, therefore, the lower the fraction which designates the lens, the higher is the power of magnification. For general purposes, "the smaller the front lens, the higher the power," is a good guide for the student to enable him to know which is the high power and which the low. It should also be remembered that, as a rule, the higher the magnification, the less working distance, the less field, and the less illumination. (Fig. 434.)

The mirror is attached to the mirror bar beneath the stage, and is freely movable. It consists generally of two surfaces, a plane and a concave, the latter of which is generally used; it should be of ample width, at least two inches.

The condenser, when present, is situated just below the stage, and if of the Abbé type, is well represented in the illustration; it is used to give a greater supply of light to the objective, and is invaluable in working with stained specimens, which are to be differentiated by means of color rather than by outline. For the best work it is an essential adjunct to the microscope. (Fig. 431.)

CHOICE OF A MICROSCOPE.

In choosing a microscope for use in the pharmaceutical laboratory, a few cardinal principles may be borne in mind. As many differences of opinion will be found among working microscopists, the question must be left to optical principles; for any microscopist, after working many years with a particular type of instrument, may become so proficient in its use, that, notwithstanding radical errors in its construction, good work can be done. Individual bias should not, therefore, govern the purchaser.

Firmness and solidity are virtues in a microscope, and as these qualities are given by the base, the pillar, and the arm, these parts should be strong and solid, not necessarily massive. We

prefer the so-called Continental type, as giving compactness, firmness, and solidity.

The stage should be ample; a square is perhaps preferable, and it should be thick and firm. The pharmacist will often use corrosive substances in his investigations, and the stage would better be of metal, not of gutta-percha.

A coarse adjustment, preferably a rack and pinion, that will not wobble, and that will not allow the tube to descend of its own weight, is a requisite. A fine adjustment is an essential; it should work evenly and smoothly, and is preferably placed at the top of the pillar.

A nose piece, double or triple, is now considered a necessity. Care should be taken that it centres the objectives, and that its joints do not leak.

As the objective is the most important part, a few words regarding its choice may be of benefit. Two errors of construction should be borne in mind; these are chromatic and spherical aberration.

Chromatic aberration is a defect due to the double action of a lens, by which it acts as a magnifier and as a prism. Acting as a prism, it decomposes the light into its elements, and if a lens shows bands of color around the edges of the object looked at, it has not been totally "corrected" for chromatic aberration.

Spherical aberration is a second defect whereby all points of an image are not brought together, so that in looking at a piece of fine wire netting, for instance, the fibres in the centre are straight and distinct, while those on the outside are found to be curved and blurred.

A certain amount of spherical aberration is necessary in high power objectives, but by means of the diaphragm the outer portions can be cut out of view.

Most modern objectives are carefully corrected for both these errors, but they should be remembered by the purchaser.

Good lenses should also possess good defining power, that is, the image should be distinct, especially at its outer borders; the diaphragm has much to do with the distinctness of the image, and care should be taken to limit the amount of light when one is testing a lens for its defining power. Penetrating power renders a lens capable of seeing clearly into an object.

Flatness of field must vary with the magnifying power and angle of aperture of the lens. A flat field is one in which all the parts of the field are in focus at the same time. Regarding the angle of aperture little need here be said, save that it represents the pencil of light that the lens is able to take in and use in forming an image; and that the angle of aperture of a lens more clearly tells its powers than the tables of magnification. Wide-angled lenses that are properly corrected are to be preferred.

Good working distance is of great importance to the micros