Purchasing the Right Microscope for Your Practice
Barry T. Mitzner, D.V.M
Most veterinarians would rather do anything than look through a microscope. I find this somewhat alarming in light of the microscopes usefulness in clinical practice. The current nature of veterinary medicine dictates that the practitioner must use all the tools at his disposal; so why the lack of enthusiasm?
Few veterinarians have methodically approached the task of purchasing a microscope and even fewer have been properly trained in its use. Many clinic microscopes are carryovers from college days or, worse yet, hand-me-downs. If they were purchased outright, its a good bet that many selections were made on price, alone. With regard to training, the majority of us received our introduction to clinical microscopy back in high school when we were handed an instrument and told to use it. If we were lucky enough to receive any formal instruction, it was sparse at best. As a result, the microscope is often the most maligned and improperly used instrument in our in-house laboratory.
There is some good news, however, for the veterinarian who is considering the purchase of a quality microscope. During the last decade, there have been a number of industry changes. While microscopes have traditionally been manufactured in Germany, Japan, and the United States, acceptable products are now coming out of Korea, Malaysia, and especially China. There have also been significant strides in optical quality. All of this translates into broader choices, higher quality, and better value for the buyer.
Another change, certainly advantageous to the shopper, involves the development of industry standards. Most microscope tubes now have a standard length of 160mm. The objective length, the distance from the object to the shoulder of the objective is also showing signs of standardization. The sizes of slides and cover glasses are uniform, at least in this country. Even immersion oil must now conform to certain specifications.
One recommendation I routinely make to veterinarians is that they own two practice microscopes: a not-too-sophisticated model for performing parasite studies, and a better instrument for cytological work. I have seen many excellent microscopes ruined by the corrosive materials used for fecal and heartworm testing. Whats more, having a separate microscope for high quality work will allow the doctor to place it in a quiet corner, away from the mainstream of activity and restrict its use to only those individuals who will take proper care of it.
The type of microscope that the veterinarian will be purchasing is called a compound microscope, so named because it consists of a combination of lenses that form the image. The stand and body of the microscope is one of the initial considerations. These parts need not be elaborate, but should be strong and stable since their job is to support the delicate optics. The nosepiece, which holds the objectives, should rotate easily and provide ready access to objectives for easy cleaning. I recommend purchasing a microscope with a quadruple nosepiece.
The microscope head provides support for the oculars and can be straight or inclined; monocular, binocular, or trinocular. An inclined head will point the ocular back towards the user and will prevent the individual from having to stand over the microscope to look through it. Most of us prefer a binocular head since it allows for more comfortable viewing over a longer period of time. Photomicrography can be interesting and can produce excellent teaching aids for clients and staff. If the practitioner anticipates this possibility, than a trinocular head should be purchased at the outset instead of a binocular head. Camera attachments can be added later as finances allow. A trinocular head will also allow for the later addition of a second head for dual viewing; however, nowadays a video set-up is a more cost effective alternative.
Objectives are by far the most important consideration in choosing a microscope. The objectives are responsible for forming the immediate image that will subsequently be examined by the ocular. There are three basic types of objectives, so named because of their degree of correction. They are achromats, semipochromats, and apochromats. The achromatic objectives comprise nearly all of the objectives in common use. The other categories are primarily for research and high quality photomicrographic purposes. The numerical aperture (N.A.) is a measure of the resolving power, i.e. the higher the N.A., the greater the resolution. Achromatic objectives tend to have moderate N.A.s though they are still quite adequate for clinical use.
A special type of achromat objective is the plano or planachromat. These are also referred to as flatfield objectives. They consist of a group of lenses that essentially flatten the image allowing elements at the periphery of the field to be in focus simultaneously with those in the center of the field. While planachromats were once necessary for rapid scanning of blood smears and the like, many practitioners are instead choosing high quality achromatic objectives and achieving very good results. Planachromatic objectives are still recommended for photomicrography. Veterinarians will need to have the following objective powers: 4X for scanning, 10X, 40X high/dry, and 100X oil immersion.
The ocular is used to examine the real image formed by the objectives. The most common type of ocular is the Huygenian, or negative ocular. Oculars must be compatible with the objectives in use, so be cautious about buying objectives and oculars from different sources. Wide-field objectives will encompass a larger area than the standard type and are recommended for longer study sessions, as they tend to reduce fatigue. High-eyepoint oculars are for individuals who need or prefer to keep their eyeglasses on while using the microscope; however, non-eyeglass wearers may find these advantageous as well.
The most common type of condenser is the two-lens Abbe type. This type will be found on most microscopes offered to veterinarians. It is important to check the N.A. of the condenser, as it should be equal to or greater than the N.A. of the highest power objective. The reason for this is that the N.A. or resolving power of the system will be no greater than the N.A. of the highest power objective. This is especially important for objectives with N.A. greater than 1.0. In order to obtain the highest resolution from these objectives, a condenser of 1.0 or greater must be utilized and the condenser must be raised so that it contacts the bottom of the slide. Otherwise, air, which has an N.A. of 1.0, will be part of the systems, relegating it to a maximum resolution of 1.0.
Stages should be of the mechanical type and operate smoothly. Left or right-handed stages are generally available.
Traditionally, low-voltage tungsten lamps have been used as the light source for microscopes. Many manufacturers have now begun replacing these with higher quality quartz-halogen (or white-light) lamps, which are recommended especially for photomicrography. The light source can be in-base or separate and should be of the adjustable type. Some systems will have a separate transformer unit allowing for even greater variation in light intensity.
Discs that impose an image of a net, scale, or crosshairs over the viewing area are called reticles. Many of use know these as micrometer discs. Depending upon the intended use of the microscope, veterinarians may want to purchase one of these. It is desirable to have the reticle mounted in a separate ocular that can be removed and replaced with a non-reticle assembly for those times when the scale is not needed.
Some general tips on shopping: Consider your needs, the number and types of samples that will be viewed, and how many people will be using the microscope. Take time to carefully review manufacturers literature or, better yet, attend a microscope or scientific instrument show. Attempt to narrow your choices down to 2 or 3 models and spend some time working with these, utilizing your own slides. Never, never buy on price alone. The less expensive model with mediocre performance will end up a chore to use and will subsequently gather dust. The best choice will often be neither the least nor the most expensive.
After you do purchase, make sure that the supplier acquaints you with the proper care needed for your microscope and the appropriate professional maintenance schedule. Only high quality lens tissue should be used to clean objectives and oculars. Recommended solvents, when needed, are methanol, or specially formulated lens cleaners. If xylene is used, use it sparingly as it is capable of dissolving some of the adhesives that are used to bind objectives together. Microscopes should be kept covered when not in use and should be protected from excessive moisture and heat. For most clinical situations, cleaning and instrument adjustment will be needed every six to twelve months.
The clinic microscope, like your spouse, constitutes a choice that you will live with for many years. The informed and careful shopper will be rewarded with an excellent and profitable investment.
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