2l6 PART II. — VEGETABLE HISTOLOGY. 



MEASUREMENT OF THE MAGNIFYING POWER OF THE MICRO 



SCOPE. 



In the practical work of vegetable histology it is important that the student 

 should know the magnifying power of his microscope, and be able readily to 

 determine the dimensions of a microscopic object. Some directions regarding 

 micrometry will therefore not be inappropriate here. We will suppose the 

 student is provided with such a micrometer as has already been described. 

 The whole length of the scale is one fifth of an inch, which is very nearly one- 

 half of a centimeter, and, except where extreme accuracy is required, may be 

 so regarded in practice. The one-fifth inch is first sub-divided into ten equal 

 parts, and one of these, near the middle of the scale, also into ten equal parts, 

 so that the larger divisions may be read as fiftieths of an inch or twentieths of 

 a centimeter, and the smaller as five-hundredths of an inch or two-hundredths 

 of a centimeter. Now, let us first determine the magnification with the low- 

 power objective and low power^ye-piece, and the draw-tube closed. There are 

 various ways in which this can be done. We will first describe one which does 

 not require the use of a camera lucida. By means of a drop of water fasten 

 the micrometer to the centre of a glass slip, put it upon the stage, and focus 

 upon the lines. Place the rule (a common foot rule will answer) parallel to the 

 scale alongside the microscope and at a distance of ten inches from the eye. 

 Look with both eyes, one directed at the micrometer scale through the tube of 

 the microscope, and the other at the rule outside. After a little practice, the 

 former scale may be seen superposed over the other and the magnification 

 readily determined. For example, if it be found that one of the larger divisions 

 on the micrometer scale correspond exactly with one inch on the rule, the mag- 

 nification is just fifty diameters. In the same way the magnification may be 

 determined for the combination when the draw-tube is pulled out one inch, two 

 inches, etc. ; and in like manner for any other combinations of objectives and 

 eye-pieces. 



A more accurate way, however, is to use a camera lucida, and project an 

 image of the scale on paper at a distance of ten inches from the' eye, or more 

 strictly speaking, from the eye point of the microscope. If the Abbfe form of 

 camera lucida be used, it should be arranged so that the whole distance from 

 the central aperture of the prism in a straight line to the axis of the mirror, 

 which is supposed to stand at an angle of 45 , and from there perpendicularly 

 to the drawing table, is ten inches. Draw the image of the micrometer lines as 

 projected on the paper and compare the spaces with those on the rule as before. 



After having carefully determined the magnification of each optical com- 

 bination, place the results in tabular form for future reference. 



By projecting the image of an object upon that of the micrometer scale, 

 previously drawn by means of the camera, its actual size may be accurately de- 

 termined ; or it may be determined, though less accurately, by placing the points 

 of a compass in front of the microscope at a distance of ten inches from the eye, 

 looking at the object through the microscope with one eye and at the compass 

 points with the other, and adjusting the latter so as to accurately subtend the 



