S. E. Dowdy. M. P. S.

Leaving this brief outline of the instrument's history, we will presume the reader unacquainted with microscopy and on the lookout for a micro-scope, and trust that the following suggestions as to the selection of a suitable one may give the chooser some idea of what to expect and what to avoid.

In the first place, the beginner should clearly bear in mind that the real benefit to himself and science derived from his taking up microscopy as a hobby will depend more on his own efforts than on the cost of the instrument. Leenwenhoek, with his crude single lenses fastened in metal plates, made observations and recorded details which the modern scientist with his carefully corrected compound lenses cannot dispute or even n some cases enhance ; and this is mentioned not to disparage modern instruments or methods, but simply to point out that care and patience can accomplish much, even when handicapped with inferior tools.

The choice of an instrument must necessarily be governed by the length of the purchaser's purse; but the average cost of a good working microscope for students' use may be taken as about $25. They can be obtained for as low as $10, or even less, second-hand. The beginner, however, is strongly advised not to purchase a second-hand instrument, unless it is a recent model by a well known maker, as unless he can obtain a friend's advice on the subject he may find himself burdened with an old-fashioned type of stand, with obsolete fittings and adjustment.

So rapid have been the advances and improvements in the cheaper forms of microscope of late years that the better plan, if unacquainted with the subject, is for the beginner to put himself entirely into the hands of a good dealer, stating what he feels disposed to invest in the purchase of one, and relying on the dealer's honesty to supply him to the best of his ability.

In purchasing either a new or second-hand instrument, above all things do not be carried away by the glamor of lacquer and highly polished metal; in fact, don't judge a microscope by its appearance, only from its performance. Lacquer cost little, careful workmanship in fitting and adjusting a lot, and the fine qualities are not always found in the same microscope. The general appearance of the microscope is, from its frequent appearance in optician's windows and similar situations, tolerably familiar as far as its external features are concerned, so that a description is not necessary. Though similar in their general appearance, microscopes may really be divided into three classes. There is, for instance, the full sized instrument, replete with every convenience that experience can suggest and wealth and skill provide. Then, again, there is the smaller edition, shorn of most of the mechanical conveniences found on the larger stands. It is with this class of microscope, generally known as the Student's, that we have to deal. Lastly, there are those microscopes which to all intents and purposes are merely toys, useless for either recreation or study. I am glad to say they are mostly importations from the Continent, their low cost being only equalled by their inferior performance. These must not be confounded with the products of such houses as Zeiss, Leitz, etc., whose work is certainly equal to some of our best opticians, though their models may not be quite so well designed. We must not forget, too, that we are indebted largely to them for improvements in the cheaper lenses now on the market. Large instruments are not suitable, and are too costly for the average amateur, their chief advantage being that they will give a larger field of view than the students' instrument, and this is not always the advantage that might be expected.

Before actually purchasing the microscope, it would be well to bear in mind some of the characterics of a good instrument for student's use. It should be simple in construction, compact, and sufficiently heavy to insure the necessary stability. Its adjustment must work smoothly and be conveniently situated both for working purposes and for repairing if necessary. Its fittings must be of standard size, and the stand, as the instrument minus its lens is termed, should be capable of taking the accessory apparatus required from time to time in microscopical work. Providing its lenses are up to the mark, an instrument having the above qualifications can be relied upon for all ordinary purposes. Before proceeding to test its adjustments, it will be as well to briefly indicate the parts which go to make up a complete stand. First of all comes the Base. This will probably be either the Continental form, which consists of a heavy horsehoe-shaped piece of metal surmounted by a thick brass pillar, on which the rest of the instrument is jointed, or else the English form of foot, which is a tripod, the spread given to the feet insuring the stability, which in the Continental form is mainly secured by the weight of the horsehoe-The chief advantage of the Continental form is its compactness; but if desirous of taking up photomicrography or drawing with the camera lucida, the student is advised to choose the tripod base, the only objection to it being that an instrument fitted with it requires a rather larger cabinet. Most microscope bases are made after these two models, or slight modifications of them.

Next comes the Mirror. This should be double, plane on one side and slightly concave on the other, and so fitted that it can be slid nearer and away from the object. Usually it is mounted on a swinging arm, so that oblique illumination may be obtained, as the fine details of an object may sometimes be made out when direct axial light fails to reveal them, if the illuminating pencil of rays of light be sufficiently oblique.

We now come to a most important part of the stand - viz, the Substage Condenser. This may or may not be present, it all depends upon the original cost of the instrument. In its simplest form it consists of two lenses, mounted in a short fitting. The lower lens is rather large, double convex, and serves to collect divergent rays from the mirror and transmit them through the upper lens, which is smaller and plano-convex, with the plane surface next the microscope stage. The chief purpose of the substage condenser is to provide brilliant illumination of an object when using high-power objectives. Incidentally, the image is improved by the illuminating rays being brought to a focus and concentrated on the object, the resultant image being more sharply defined than if light from the mirror alone without any condensing system was being employed. If purchasing a new stand, insist on having one either provided with a substage condenser, or else a fitting capable of taking one at some future time, as they may be purchased separately at any time. If this is out of the question fit in an improvised substitute, as a microscope nowadays can hardly be called such if unprovided with such a necessary adjunct.

The Stage, as the flat plate on which the slides are placed for examination, is termed, will now require a short comment. In students' micro-scopes it is simply a metal plate, with either a circular or horsehoe-shaped opening, on each side of which is a spring clip for holding down the slide. Better class microscopes have mechanical stages where motion is imparted by rotating milled heads, but as these require very skilled labor in fitting and adjusting, they cannot very well be added to a cheap instrument. The addition of a sliding bar to the ordinary stage is an improvement, and can be obtained to fit nearly any make of stand. If the microscope is not fitted with a substage condenser, it should have a metal plate, furnished with circular apertures of varying diameters, to revolve just under the stage aperture. This arrangement is called the diaphragm, and is used to cut down the amount of light thrown up by the mirror. Very trans-parant objects with delicate detail are quite obscured if too much light is being used, and this is where the diaphram is useful.

The Body-tube next requires a little description. Two lengths are adopted nowadays by the makers; viz., 6 inches for students' instruments, 10 inches for their larger stands. It is important to note the difference because objectives are corrected nowadays for either one tube length or the other, and it is necessary, if the highest excellent of definition be desired, that a lens should be used on that particular tube length, for which it was originally corrected and intended. If possible, get an instrument the body-tube of which is fitted with a draw-tube ; a short length of tubing, the upper end carrying the eyepiece, and sliding up or down in the main tube. The draw tube is a most useful edition to a microscope as increased magnification is obtained by its use, and if a student's pattern instrument, objectives for both 6 in. and 10 in tube lengths can be used on the same stand by simply closing up or pulling out draw-tube. The diameter of the main tube may also vary. Student's stands take an eye piece of about .92 in. diameter, the large English stands 1.07in. Unfortunately, all eye pieces are not made to exactly the same gauge, as in the case with objectives mounts, so that it is as well to purchase the eyepieces from the same maker who supplied the stand.

Focussing Adjustments will now claim our attention. These, as a rule, are two in number, and are known as the coarse and fine adjustments. The former, as its name implies, is for the purpose of obtaining a rough or approximate focus, the latter for getting the correct adjustment necessary to obtain a sharp picture of the object. Coarse focussing may be accomplished by either sliding the body-tube up and down in a cloth-lined collar, this method being adopted in the cheapest stands, or a preferable method being that where a rack and pinion adjustment is substituted, whereby a more delicate motion may be imparted to the tube. It is, however, well worth the extra expense to have the rack and pinion adjustment, which if good, will enable the student to do all his focussing, except when using his highest power lens. The fine adjustment, by the aid of which an almost inappreciable movement may be imparted to the optical system in the microscope, is one of its most indispensible adjuncts. High-power work would be quite impossible without its help, so sensitive are modern high-magnifying objectives to the slightest alteration of their distance from an object. The necessary motion is imparted by rotating a milled head, which in its turn actuates a lever or rotates a screw, different makers adopting different methods to accomplish the same end. As it is important that both coarse and fine adjustments should be of first-class workmanship, their behavior will be noticed when we are testing the lenses supplied with the microscope.