The questions are seen in the given file

WORKSHEET FOR LAB EXERCISE 1: MICROSCOPES AND MAGNIFICATION

1. MICROSCOPES

Organisms and other objects are studied with the use of magnifying instruments that provide enlarged images of the object. A simple biconvex hand lens (of two or three glass parts) can magnify up to 10 or 12 times. For magnification higher than these, a microscope is necessary.

1. 1. COMPOUND MICROSCOPE

A compound microscope consists of certain precise mechanical parts (chiefly of metal) to support and facilitate the use of the optical parts (of glass) that provide the magnified image.

1. 1. 1. Mechanical Parts

         1. Base –foot on which the microscope stands; acts as microscope supports; also carries the microscopic illuminator (light source)

         2. Light switch – switch on the base of the microscope that turns the illuminator on and off

         3. Brightness adjustment

– used to vary the light that passes through the stage opening and helps to adjust both the contrast and resolution of a specimen

1. 1. 1. 1. Stage – platform with central aperture and clips to hold the slide being studied

         2. Stage clip – holds the specimen slides in place

         3. Stage controls or knobs – control the movement of slide: one knob moves the slide left and right; the other knob moves the slide forward and backward

         4. Aperture – the whole on the microscope stage, through which the transmitted light from the source reaches the stage

         5. Diaphragm – also known as the iris, is a metal plate with circular openings with different diameters that control the amount of light reaching the object

         6. Coarse adjustment knob – outer rotary knob that quickly moves objectives to focus or obtain an image of the object

         7. Fine adjustment knob – inner rotary knob used for more delicate focusing especially at high magnification

         8. Arm – a handle used in carrying the microscope

         9. Nosepiece – also known as the revolving turret; bears the four objective lenses

         10. Head – also known as the body, it carries the optical parts in the upper part of the microscope

1. 1. 1. Optical Parts

These consist of special types of glass, carefully ground and polished, and aligned on an optical axis.

1. 1. 1. 1. Illuminator or light source – the light source for a microscope located at the base, which captures light from an external source of a low voltage of about 100v; it is used instead of a mirror. Older microscopes used mirrors to reflect light from an external source up through the bottom of the stage.

         2. Condenser (lens system) – found between the mirror and the stage and serves to further concentrate light rays on the specimen

         3. Objectives of object lens – consist of two or more lenses fixed in a rigid mount that serve to form a real image of the object within the body tube

            1. Scanner – smallest and shortest, with lowest magnification but allows viewer to see a greater area

            2. Low power objective (LPO) – shorter lens with a magnifying power of 10x

            3. High power objective (HPO) – the longer lens that magnifies 40x and forms a bigger image of the object

            4. Oil immersion objective (OIO) – the longest objective, with a magnifying power of 100x, used only with immersion oil

         4. Ocular or eyepiece – two larger lenses at the

top of the draw tube that serve to further magnify the image. The lens of the ocular refracts (bends) the light rays passing from the real image to the retina of the eye in such a way as to produce the effect of a still virtual image (ghost image). The latter is imaginary (it cannot be projected on a surface); it produces on the eye the same effect as if an object and the size of the virtual image were held at ordinary reading distance.

2. What has the microscope done to the size?

Based on the images in Plate 1, answer the following questions:

1. What has the microscope done to the position of the image of the object?

3. How does the printer’s ink in the letter “e” differ in appearance from what is seen with the unaided eye?

4. What is the effect of the shifting from LPO (100x) to HPO (400x) on the size of the specimen?

5. What is the effect of the shifting from LPO (100x) to HPO (400x) on the area covered or seen as field of view?

1. 1. BINOCULAR DISSECTING MICROSCOPE
      

Certain exercises in this manual call for the use of a dissecting microscope. This microscope combines lower magnifications with binocular viewing and is of considerable value in making delicate dissections of small specimen. The techniques involved in the use of this instrument are basically similar to those used with the compound microscope.

This instrument has paired objectives and oculars to provide binocular (two-eyed) vision, whereby relation of parts at different levels in a specimen may be determined. It contains a series of prisms that produce an erect “image” (not inverted as in compound microscope). The magnification is about 5 to 50 times only, depending on the objectives and oculars supplied, and there is only coarse adjustment for focusing. Means are provided to adjust the two oculars for the distance between the eyes (interpupillary distance) of the user. The tube supporting the ocular may be focused separately to compensate for differences between the eyes of the user.

Some parts of the binocular dissecting microscope are similar to that of a compound microscope.

1. 1. 1. Mechanical Parts

         1. Base – foot on which the microscope stands; acts as microscope supports; also carries the illuminator (light source); also acts as the stage on which object/specimen is placed for viewing

         2. Stage plate – a clear glass plate located above the light source (bottom light) to allow light to pass through and reach the specimen; located at the central, front part of the base

         3. Stage clip – holds the specimen slides in place

         4. Arm – may be described as the “backbone” of the microscope given that it provides support to the head part of the microscope by connecting it to the microscope base. It is where the power cord for illumination is located, or originated from, the uppermost. It is also where the focus knob can be found.

         5. Light switch – switch on the base of the microscope that turns the illuminator on and off

         6. Focus knob – also known as the coarse knob, moves the head of the microscope up and down to bring the object sharply into view; located on the arm of the microscope

         7. Zoom knob – used to zoom in on to a particular area of interest on the field of view to get a closer look; located on the side of the microscope head just below the eyepieces

         8. Stereo head – carries the optical parts in the upper part of the microscope

         9. Cord – the power cord which is normally located at the top of the arm

1. 1. 1. Optical Parts

         1. Illuminator – source of light (bottom light) located at the base

         2. Objective lens – two simple convex lenses of different magnifications (magnification power ranging from 1x to 4x) located in a cylindrical cone and are therefore not directly seen

         3. Binocular or eyepieces – two eyepieces each focusing different pathways of the light into and out of the specimen, each with its own magnification power

1. 6. What is a stereoscopic image?

* Refer to image of the human head louse as seen under the dissecting microscope (Plate 1.2) to answer Table 1.1 below.

Table 1.1: Observations on the human head louse image

7. How would you compare the image seen under a dissecting microscope with that of CM? Refer to Plates 1.1 and 1.2 and write your answers on Table 1.2 at the next page.

Table 1.2: Comparison of the image(s) seen under the microscopes

1. MAGNIFICATION

Magnification means the number of times an object is enlarged or reduced by a lens system of the microscope or by a drawing.

1. 1. MICROSCOPE MAGNIFICATION

In microscopes, magnification is achieved by a system of biconvex lenses situated in the ocular and objective. The objective lenses magnify the specimen to an enlarged first image (real image) and then, the ocular lenses magnify the image further to a larger second image (virtual image). As a result, the image seen by the eye has magnification equal to the product of the magnifying power of the two lens systems. The total magnification in either a compound or a binocular microscope is known as Linear Magnification. To calculate the linear magnification, we simply multiply the ocular lens magnification by the magnification of the objective lenses using the formula:

Where: LM = linear magnification

OcM = magnification of ocular used ObM = magnification of objective used

NB: The product or numerical value of the linear magnification is always written with a letter “x” before or after the number, e.g. 10x or x100.

1. 1. MAGNIFICATION OF DRAWING

Whenever a drawing is made in a research paper, some indication of its size must be given. This is known as magnification of a drawing. The magnification value tells the viewer how much larger or smaller the drawing is than its actual size. The magnification of a drawing is determined by using the following equation:

Where: M = magnification of the drawing

SD = size of the drawing

SO = actual size of the object

B.1. Computing the Magnification of Drawing:

Thus, if the actual length of the specimen is 10 mm and the drawing is 100 mm, we divide 100 mm by 10 mm, and the resulting quotient is 10, which indicates that the drawing has enlarged the specimen image by 10 times. Since during arithmetic calculation, the units of measurement “mm.” are canceled out, we write “x” before or after the quotient to indicate that the computed quotient or value is a power of magnification. This is written as “x10” or “10x” and is written after the caption of the drawing in scientific reports. The caption of the drawing is always written below the drawing with a prefix “Figure” or “Fig.” and the number of that figure in the research paper.

B.2. Computing the Actual Size of a Microscopic Specimen

Computing for the actual size of the microscopic object, two values are needed: the calibration constant of the microscope (Cal.k) and the number of eyepiece micrometer spaces occupied by a focused specimen under the microscope. Since the calibration constant is typically predetermined (which, by our Leica microscope, is 0.001 mm (LPO and 0.0025mm (HPO.), the calculation of the actual size of a microscopic image is simply the product of the above-mentioned values. To find the actual size of a microscopic specimen, the eyepiece micrometer must first be inserted inside the ocular lens system. Then, the microscopic specimen is focused under the microscope with the division marks of the eyepiece micrometer superimposed on the specimen image. Then, the number of eyepiece micrometer divisions or hash marks occupied by the specimen is counted. The actual size of the specimen is computed by multiplying the calibration constant (in millimeter) and the number of eyepiece micrometer divisions occupied by the specimen under the microscope using the formula:

Where: SO = actual size of specimen (mm)

C = calibration constant

sd = number of eyepiece micrometer spaces occupied by the specimen

Reflections / Generalizations:

END OF LAB EXERCISE 1 WORKSHEET