. The microscope; a simple handbook. Microscopes. 80 THE MICROSCOPE Table ot drawtube corrections. < Length of Drawtube. 140 mm 160 mm. 160 mm. 180 mm. 200 mm. 240 mm. Corresponding Cover Glass Thicknesses. in. mm. in. nun. in. mm. in. mm. in. TnTH- in. mm. liin. (32mm.)i . 2/3 in. (16 mm.)i. 1/3 in. (8 mm.)i . 1/6 in. (4 mm.) . â 200 â¢100 â¢018 â¢008 5 2-5 â¢46 â¢2 â¢140 â¢085 â¢012 â 0074 3^5 21 â¢31 â 19 â¢100 â¢070 â¢007 â¢007 2^5 1^8 â¢17 â¢17 â¢070 â¢050 â¢004 â 006 1^8 13 â 1 â¢15 â 040 â¢030 â¢0053 â¢1 â 08 â 135 â¢004 â¢1 1 1 The lower power lenses selected for the purpose of comp
. The microscope; a simple handbook. Microscopes. 80 THE MICROSCOPE Table ot drawtube corrections. < Length of Drawtube. 140 mm 160 mm. 160 mm. 180 mm. 200 mm. 240 mm. Corresponding Cover Glass Thicknesses. in. mm. in. nun. in. mm. in. mm. in. TnTH- in. mm. liin. (32mm.)i . 2/3 in. (16 mm.)i. 1/3 in. (8 mm.)i . 1/6 in. (4 mm.) . â 200 â¢100 â¢018 â¢008 5 2-5 â¢46 â¢2 â¢140 â¢085 â¢012 â 0074 3^5 21 â¢31 â 19 â¢100 â¢070 â¢007 â¢007 2^5 1^8 â¢17 â¢17 â¢070 â¢050 â¢004 â 006 1^8 13 â 1 â¢15 â 040 â¢030 â¢0053 â¢1 â 08 â 135 â¢004 â¢1 1 1 The lower power lenses selected for the purpose of compiling this table are not as usually supplied, but are specially corrected for longer tubes to show the variation better. Canada balsam acts in a similar manner to glass, and a layer of Canada balsam between the cover glass and the object has the efEect of increasing the thickness of the cover glass. It must be allowed for unless the object is mounted in contact with the under-surface of the cover glass., Correction of It will occur to the reader that the correct tube length can tototaeffi'by be ascertained by observing the image through the microscope. obserration. ipjiig jg ygry readily done under dark-ground illumination, and with more difficidty with transmitted light. Under dark- ground illumination there wiU always be fine specks of dust illuminated as brilliant points; one which is so small as to show no apparent outline or shape should be selected and be placed near the centre of the field of view. It should then he focussed backwards and forwards, and the image examined on each side of the sharpest focus. If the light is aU coming to an exact focus at any one point, the appearance on each side of the focus will be the same : it will be a small disc or patch of light equally intense. on either side. If it is out of adjustment, one side wiU be clear and the other side hazy. Fig. 89 shows that if aU the light. Please
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectmicroscopes, bookyear
