THE COMPOUND MICROSCOPE

[Music] this motion picture is about the compound microscope it looks like this if you work in almost any field of science you will want to know how the microscope works and what kind of a job it can do in capable hands there are many different kinds of microscopes this one equipped with a binocular body is suited for long periods of observation this model is designed to convert to binocular use this is also a compound microscope although a simpler model designed primarily for elementary student use and all of these while slightly older models are also efficient instruments still in daily use in many laboratories throughout the world in spite of the many different models all of these microscopes magnify effectively because of certain basic principles which they share in common we can take any one of these instruments as a starting point this one will serve as our example you will want to understand your microscope because a significant factor in your future success may be the skill with which you use this precise scientific instrument this diagram shows the path of light through the microscope as it is obviously rather complicated let's first discuss some elementary facts about vision let's assume that this is an object and this is the human eye the eye has a lens and an area sensitive to light called the retina light rays from the object travel through the lens of the eye and are focused on the retina permitting us to see the object if we now introduce a simple lens between the object and the eye the rays of light follow this path the result is an enlarged image if we should insert a second lens into this system we see a still larger image this is the sort of optical system on which the compound microscope is based and the final magnification of a microscope optical system is the product of the power of the objective multiplied by the power of the eyepiece this represents the stage of the microscope on which we place our specimen light rays pass from the specimen through the lenses of the objective and continue up the microscope tube to form a real image called a primary image this primary image could actually be seen on a ground glass screen since it is formed in the air it is also called an aerial image we are not however interested in looking directly at this image instead we look at it through an eyepiece which is in effect a magnifier the final image appears to be located in a plane below the stage of the microscope this final image is known as a virtual image to differentiate it from the real image this optical system is basic to all modern compound microscopes since this is the common denominator of all compound microscopes we might call the optical system the heart of your microscope so that this complex optical instrument may function for many years at its maximum efficiency it must be designed constructed and assembled to precise specifications the microscope has a base the function of which is to support the rest of the microscope this particular instrument has the fine adjustment controls located in the base for ease of critical focusing the arm of the microscope is attached to the base at a pivoting joint so that it can be tilted as desired the other mechanical and optical components of the microscope are attached to the arm here is the stage of this microscope it has these holes drilled into it on the side these permit attaching a mechanical stage by means of two thumb screws the mechanical stage can be moved smoothly in two directions by means of racks and pinions controlled by two adjustment knobs most mechanical stages are equipped with a movable finger this one is controlled by a vacuum spring combination when released the curved finger applies a three-way pressure against the specimen slide down across and back this action maintains correct positioning of the slide during movement of the mechanical stage the body tube is held to the arm by a rack slide assembly which also includes the course focusing mechanism the bearing surfaces are fitted with precision so that they move against each other almost without friction oil should not be used on these bearing surfaces the revolving nose piece is attached to the lower end of the body tube with openings for two three or four objectives the most recent microscopes of this particular type have nose pieces which are designed with pressure loaded ball bearings around the outside rim thus distributing the weight load throughout the circumference and decreasing wear objective is marked with its focal length in millimeters its magnifying power and its numerical aperture commonly called an a the higher the power the more complex is the optical system of the objective the eye pieces are fitted to slip into the top of the body tube being exposed to the air they must be kept scrupulously clean most medical microscopes are supplied with both 5 power and 10 power eyepieces beneath the stage of this type of microscope is the condenser its function is to form the light that comes from the mirror into a cone suited to the objective in use there are many different kinds of condensers conventional abbey type can usually be divided by removing the top lens element for use with objectives of 10 power or less the complete condenser is used with objectives of higher power the variable focus type is a modern variation of the abbey in the case of the variable focus condenser the cone of light entering the objective lens is buried by racking the lower condenser lens up or down the effect of this operation can best be demonstrated by placing a block of fluorescent glass on the stage and racking the condenser up and down as the lower lens approaches the stage the angle of light tends to approach the horizontal which is essential for objectives with high n a such as the 97 power oil immersion objective as the condenser is lowered toward the mirror however the light rays tend to become more vertical thus it is clear that for objectives with higher magnifications and higher n a the condenser must be up toward the stage the higher the n a of the objective the more nearly horizontal the light must be to enter it the more accurately this is done the sharper the image will become ideally the condenser should be capable of matching the n a of each objective on the microscope the major function of all microscope condensers is to provide suitable specimen illumination to permit objectives to achieve their maximum resolution the variable focus condenser has been found to achieve this function most completely the mirror is used to direct light from the source upward into the microscope system this one is attached to the microscope by a tuning fork bracket which provides rigid support for the mirror and permits easy removal if desired some microscopes designed for elementary work at low powers do not require condensers for this work a curved mirror provides the necessary illumination but when a mirror and condenser are used together the flat side of the mirror is always used with the addition of the mirror the microscope is complete an essential piece of equipment for using your microscope is a suitable source of illumination the ideal source for advanced microscopy has an adjustable diaphragm for reducing the size of the beam of light and equipment for adding filters to control the color or intensity of the light which reaches the microscope a popular form of illuminator is the substage variety which can be used in front of the mirror on its back under the microscope stage or attached by means of a bracket to the microscope in place of the mirror as an integral part of the microscope this is another simple illuminator providing greater light intensity it is generally satisfactory for routine microscopy assuming that we have a satisfactory illuminator we place a specimen slide on the stage after the slide is in place the course adjustment is used to bring the low power objective down to approximately one quarter inch above the slide while looking into the eyepiece the observer uses the course adjustment to raise the objective until the specimen comes into approximate focus critical focus is achieved by careful use of the fine adjustment here the 10 power objective a low power objective is used to survey the slide for various areas which might warrant more complete study at higher magnification when such an area is found the 43 power objective is turned into place in the case of recent microscopes equipped with par focal objectives only a slight adjustment of the fine focusing is required to bring the specimen into sharp focus in the case of microscopes which do not have par focal objectives the body tube is raised the 43 power objective is turned into place and the objective is carefully lowered to the slide the focusing is then completed by turning the fine adjustment upward not downward as that might result in cracking the slide and damaging the objective lens you will notice that when the center of the field is in sharp focus the edges are slightly out of focus on the other hand an attempt to bring the edges in focus will cause the center of the field to appear out of focus this is a phenomenon common to any highly magnified field and increases with the higher magnifications after the slide is in focus it may still be difficult to distinguish structures clearly because of poor resolution under high magnification adequate resolving power should be obtained in the following manner first remove the eyepiece from the body tube use a neutral density filter or a piece of heavily ground glass to protect the eyes from the intense light look through the tube at the objective second open the substage diaphragm fully third move the condenser up or down until the back lens of the objective is filled with light when this occurs the condenser is adjusted so that its focal length and n a are matched to that of the objective being used frequently it is desirable to close the substage diaphragm until the back lens of the objective is about two-thirds filled with light this reduces stray light thus improving image contrast fifth replace the eyepiece and resume observation you will recall that the specimen looked like this before adjusting the condenser after making the adjustments outlined in the previous five steps the same area of the specimen slide looks like this it is obvious that we have obtained much better clarity of image in many cases the clarity of the image can be improved by reducing the intensity of the light reduction of the light intensity should always be done at the illuminator never by means of the substage diaphragm on the microscope before discussing the specific technique for using the oil immersion objective it is important to understand why immersion oil is necessary this objective has such a high n a that the rays of light passing through air are either reflected from the front lens surface or pass outside the edge of that lens by using oil on the top and bottom of the slide we can exclude the air the oil used has optical properties similar to the glass of the objective and permits the light to enter the objective at the proper angle after the approximate position of the area to be observed has been located a drop of oil should be placed on the top lens of the condenser or on the bottom of the slide for speed and ease of operation some microscopists put oil only on the top of the slide this however is done at some sacrifice of image quality second the field is carefully surveyed using the 10 power objective and the particular area of interest is moved to the center of the field third the 43 power objective is turned into place and by using the fine adjustment the image is brought into focus in the center of this much smaller field then turn the 97 power objective into place the fifth step is to place a drop of oil on the top of the slide the objective lens is observed from the side of the stage the objective is carefully lowered until contact is made with the immersion oil when this contact is made a brief flash of light will be seen the last step in focusing is taken while looking through the microscope using the fine adjustment and focusing cautiously up and down the specimen is brought into sharp focus the special oil used on the oil immersion objective should be kept covered to prevent dust accumulating in it and being transferred to the lens after being used the front lens of the oil immersion objective and other optical parts which have been oiled should be carefully wiped dry with lens paper solvent should not be used as a rule but if it is necessary a common solvent such as tolual or xylo should be applied very sparingly regardless of the particular kind of equipment your microscope may have the basic rules of care and cleanliness can be summed up in one phrase keep it clean microscopes last a long time because looking through them doesn't wear them out but they can be damaged by improper handling the microscope should be protected from dust and dirt if the rack and pinion adjustments of the microscope become stiff these bearing surfaces should be cleaned with a small amount of xylol and then re-lubricated so that they operate smoothly lubricating oil should not be used on the bearing surfaces only an occasional thin coating of vaseline should be used the eye pieces and objectives should always be carefully inspected and properly cleaned before being used they should be handled carefully so as not to smudge the optical glass with finger marks kept clean and in good working condition the microscope will last a lifetime as you learn to use your microscope with increased facility it will acquire ever greater importance as a factor in aiding you to observe distinguish and interpret an otherwise invisible world [Music] you