To test an oil immersion or water immersion lens it should be tried upon stained bacteria, or suitable diatoms, but as such a lens does not comprise part of an ordinary students' outfit, we can leave the matter until the observer has sufficient microscopical experience to suggest the best method for such an objective. It will then be time enough for thinking of purchasing such an article. We have now shown how the lenses supplied with the microscope, or purchased subsequently, may be tested, and by the time the student has done this, he should have arrived at the best method of handling the instrument for ordinary working purposes. A few words, as to certain precautions that should be used in actual working may not be amiss at this point.

It is often thought that work with the micro-scope and artificial illumination ruins the eyesight. Such is not the case, if care be taken to only use sufficient light to clearly make out the details of an object, to only work when in good physical condition, and only so long as no strain is felt on the eyes. The yellow glare from the lamp employed should be toned by passing it through blue glass, and this can be done by either using a blue glass chimney, or by placing a disc of the glass in the stop-carrier of the substage condenser. Powerful eyepieces should be avoided as much as possible, and only sufficient power used to clearly make out the detail looked at-It is also a good plan to get into the habit of using the right and left eye alternately, and to keep the one open whilst the other is at the eyepiece. This may seem difficult at first, but is easily acquired by a little practice. It has been noticed that whilst the eyes of one accustomed to use the microscope to a great extent may be rendered less susceptible to intensity of light, they become much more sensitive with regard to perceiving fine detail, which is only what might be expected. With regard to using both eyes, the beginner may be tempted to say. "Why not purchase a binocular microscope, and always use both eyes? Unfortunately, however, binoculars cannot be used to much advantage except with low-power objectives, owing to loss of sight from the prisms and other optical reasons. Some of the best class of students' stands can now be obtained fitted with binocular body-tubes, but the novice is not advised to purchase one for reasons too numerous to mention to be gone into here, though, if he likes to obtain one in which the monocular body can be easily substituted, there will be occasions on which it can, as a binocular, be used to advantage.

Having described the instrument and its lenses, a word or two with regard to the use and purchase of the various adjuncts to it, known as accessories, may be said. The student will not be long in finding that to successfully carry out many operations with the microscope he must employ one or other of these adjuncts. For instance, if he wishes to look at an opaque object, as such, he must have recourse to the use of what is known as a "bull's eye condenser," or if he wishes to rapidly change his lenses he must, unless he unscrews them every time, employ a nose piece, and so on.

Accessories can always be purchased separately or picked up second-hand, and such should include, besides the bull's eye condensers and nose piece just mentioned, a live box, micrometers, a cheap form of camera lucida, and some form of olavising apparatus. For mounting objects a pair of forceps, scissors, watchglasses, slides, covers, and the usual paraphernalia will be requisite, besides a good form of hand lens, either the ordinary single lenses in frames or, preferably a compound lens, such as those termed aplanatic, etc., by the makers.

Space will not allow of more than the briefest indication of the use of each of the more generally required accessories, and the beginner is advised to first procure as good a textbook on microscopes as he can afford, and to carefully make out the specific use and scope of each article before purchasing it; otherwise he may obtain his experience in a not unusual fashion by paying rather dearly for it. I would, however, warn novices from buying old and out-of-date works on the microscope, as in the light of recent improvements and discoveries they are only misleading, and apt to create erroneous impressions. For the benefit of those quite ignorant of the use of the microscope and its accessories I might briefly describe those adjuncts which I have mentioned as being required to complete the student's microscope outfit.

The Bulls-Eye Condenser. - This useful piece of apparatus consists of a plano-convex lens, mounted in a sliding arm attached to a pillar so that any required position of the lens may be attained with a little manipulation. The specific purpose of this accessory is to illuminate opaque objects by concentrating and bringing to a focus light from the lamp upon these specimens, which could not from their opacity be viewed by transmitted light from the mirror. It can only be satisfactorily used with low power objectives, so that its usefulness is to a certain extent restricted, still, it is a useful accessory for all that. Bull's-eye condensers vary in price from about $2.00 upwards, according to size of lens and method of mounting, but a small one will answer the beginner's requirements.

The Nose Piece. - This is one of the most useful and time saving devices the worker with the microscope can possess. It consists of a peculiar shaped revolving double collar, rather difficult to describe, but very simple in construction and working. By its means objectives of varying focal length can be employed and rapidly changed without unscrewing or screwing in the lenses each time into the body-tube of the instrument.

Both time in working and wear and tear on the objective screw threads are saved, thus constituting this little addition to the outfit a most wel-one. Nose-pieces are made in either brass or aluminum to carry two, three, or sometimes four objectives. The student is advised, however, to obtain one to carry two only, because all the weight in some forms of instrument comes upon the fine adjustment, and a quadruple nose-piece loaded with four objectives is no light weight for a delicate mechanism and stand. The price of this necessary ranges from $2.50 upwards, for simple as it is in appearance, it requires to be very carefully made to insure accuracy in centreing the lenses. One of its chief uses to the student would be in finding objects when high magnifying powers were being employed. For instance in picking out a certain diatom on a mixed slide, a low-power lens would be used, the diatom being put into the centre of its field; the nose piece would then be rotated to bring the high-power lens to bear upon it, when, on focussing, the diatom should appear in the centre of the field of the higher-power lens.

A Live Box consists of a raised ring of brass mounted in a small brass plate, and having a circular glass resting upon it. Over this ring another is made to slide, this upper ring also encircling a glass cover. Any object placed on the lower glass, and the cover put on, is therefore in a sort of box or cage. This piece of apparatus, costing anywhere from 75 cents upwards to $2.50 or thereabouts, is principally used in observing pond life, but. can be put to a variety of purposes. It can, however, be dispensed with, as an ordinary shallow cell can be made to answer fairly well in its place.

Micrometers consist of glass slides or circles, ruled in minute parts of an inch, or in millimeters. They are, of course, for measuring specimens viewed under the microscope, and are also of use in determining magnifying power. The methods of using them can be seen in any good textbook, and they are useful little appliances. Two would be required if the student wished to determine visually the dimensions of objects, one ruled and mounted as an ordinary 3" x 1" slide, the other as a circle to drop into the eyepiece. The usual cost is $1.00 each for those ruled to the 1/100" and 1/1000".

A Camera Lucida for the microscope is an appliance for making accurate drawings of objects. The simplest form, and the one most suitable for the student is what is called Beale's neutral tint reflector, costing about $1.50. There is a certain knack in using both forms of camera lucida; but when once picked up it is not readily forgotten, and accurate drawings, or rather tracings to scale, may be made by onyone.

Photography is, however, rapidly displacing this method of recording observations, though, of course, photography is not always applicable.