Micrometry and camera lucida

MICROMETRY AND CAMERA LUCIDA

1. MICROMETRY AND CAMERA LUCIDA

2. MICROMETRY The field of science dealing with the measurement of microscopic objects using micrometer is called micrometry or morphometrics . While studying microorganisms , it is often necessary to measure their length,breadth,diameter etc . This can be done with the help of a device called micrometer. MICROMETER Greek-micro(µ): small ,meter : a measure Micrometer is an instrument attached to microscopes for measuring dimensions of small objects. WILLIAM GASCOIGNE invented the micrometer. Robert Hooke improved Gascoigne’s micrometer by substituting parallel hairs for the parallel edges. Microscopic measurements were first performed in the late 1600s by Anton Von Leeuwenhoek, who used fine grains of sand as a gauge to determine the size of human erythrocytes.

3. Microscopic measurements are in the range of the average field diameter of wide field eyepieces i.e. 0.2µ -25µ. Measurements below 0.2µ are beyond the resolving power of the microscope. Lengths larger than the field view of wide field eyepiece are usually measured with stereomicroscopes. The measurements are expressed in mm, µm (light microscope) or nm(EM). The parameters that can be measured are : length(linear), thickness(volume), area and angles.

4. COMPONENTS OF MICROMETER 1)Stage micrometer 2)Ocular micrometer Stage micrometer is a microscope glass slide at the centre of which has a very finely graduated scale measuring 1mm.This line is divided into 10 equal divisions by small lines with longer lines marked as 0,1,2,3…and so on. Each of these divisions is subdivided into10 equal divisions by small lines. Thus 1mm is divided into 100 equal divisions and hence the magnitude of each division is 0.01mm Ocular micrometer/Eye-piece micrometer/Eye-piece reticle Ocular micrometer is a transparent circular glass disc which is placed in the body of ocular lens(eye piece) of the microscope. In the centre of the OM a line is drawn. This line is subdivided into 100 equal divisions, as in the case of the stage micrometer. Graduations of eyepiece scale are referred to as reticle. However ,the scale on ocular micrometer does not have any standard value. When the OM is placed in the eyepiece, the ruled lines superimpose the SM divisions

5. CALIBRATION OF OCULAR MICROMETER Micrometric calibration involves finding out the absolute magnitude of each division of the OM scale with respect to the magnitude of the SM division. This value is called micrometer value, or calibration factor. Calibration can be done only after super imposing the graduations of both stage and ocular micrometers.

6. 1 division in ocular = number of divisions on stage micrometer number of coinciding divisions on ocular micrometer One stage division=0.01 mm or 10µ ADVANTAGES OF MICROMETRY Best method for measuring microorganisms. Best method for taxonomic measurements.

7. PROCEDURE OF MICROMETRY Standardization of ocular micrometer using stage micrometer. Find the value of 1 ocular division( eg:25µ). Remove stage micrometer and place object slide. Measure the object using ocular scale. Calculate the actual size. e.g. Suppose mean length and breadth of a cell is 4.2 and 2.5 ocular divisions Length =4.2×1 ocular value =4.2×25 =105µ Breadth=2.5×1 ocular value =2.5×25 =62.5µ Size of the cell=105µ×62. 5µ

8. Let the no. of division on the stage micrometer be y and the no. of divisions on the ocular micrometer be x . Since each stage micrometer divisions is 0.01 mm apart, the distance between the starting point and point of coincidence on ocular and stage micrometers can be calculated at that magnification. In other words: Distance covered by x no. of OM = Distance covered by y no. of SM divisions divisions Since1SM division is 0.01 mm, the distance covered by x divisions of OM = y divisions × 0.01mm Therefore, 1 OM division = y division × 0.01mm x division

9. Suppose 10 OM division ( x ) overlaps 7 SM divisions ( y ) at 40x magnification 1 OM division= stage × 0.01 ocular = 7 × 0.01 10 = 0.007 mm Therefore calibration is the process of determination of the magnitude of one division of the ocular micrometer at a particular magnification with the help of a stage micrometer. Here the values of ocular micrometer division is calibrated as0.007 mm.

10. CAMERA LUCIDA Camera Lucida was developed by Wollaston for making drawings of microscopic objects used along with microscope. This is an accessory instrument. DEFINITION: It is a simple optical device which enable the observer to make reasonably accurate outline drawings of correct scale and proportions of objects seen under the microscope. OR It is an instrument ,mounted on the top of the body tube of a compound microscope for making clear, simple and exactly proportionate outline sketches of the objects under study.

11. CAMERA LUCIDA

12. CAMERA LUCIDA-PARTS Mirror ,Reflecting prism Mirror:Adjustable,inclined at 45º both to the desktop and the prism. Prism: surface is silvered. The particular central area that coincides with eyepiece is not silvered. PRINCIPLE : deflection/reflection of light The light rays passing through the ocular lens are deflected at 90 by the prism. These rays are further deflected at90 by the mirror. This arrangements give a virtual image on the desktop beside the microscope. If a piece of paper is kept there, the viewer can see and draw that image. Magnification is due to the angular deviation

13. ADVANTAGES Gives accurate sketch of the microscopic objects. The outline sketch is a magnified version of original object on the slide.—Tracing of outline is an easy task. HISTORY The camera lucida was patented in 1807 by William Hyde Wollaston . The basic optics were described 200 years earlier by Johannes Kepler in his Dioptrice (1611), but there is no evidence he or his contemporaries constructed a working camera lucida. By the 19th century, Kepler’s description had totally fallen into oblivion, so Wollaston’s claim was never challenged.

14. CALCULATION OF MAGNIFICATION It is computed by multiplying the power of the eyepiece and the objective lens used. Focus the stage micrometer at the center of the field of observation. Draw a few division with the help of the drawing prism For more accurate result, take 2 more drawings at the left and right side of the field of observation.(Because the magnification varies according to the position of the stage micrometer, greatest at the left and lowest at the right) Measure the drawn divisions by a mm scale Calculate magnification as follows : e.g. 1 magnification stage division= 5 mm (1 mm=1000µ) i.e.,10µ is magnified to 5x1000µ=5000µ `.` 1µ is magnified5000 =500 10 So the magnification is 500 times .