250 THE AMERICAN MONTHLY [November, 



an encouragement, I take it, to all who may be in earnest, but who for 

 some reason are unfavorably situated. Let us now see how work is 

 done with the appliances that the development of science which has 

 occurred since Leeuwenhoek's age, has placed at our own disposal. 



Retaining your microscope in the horizontal position used in draw- 

 ing, substitute your micrometer for the object on the stage. Focus 

 your microscope till the lines on the scale show clear and sharp, and 

 then project them on the paper as you would any ordinary object. If 

 your sketch is on the table, you can at once determine the exact size 

 of the various parts of the object delineated by projecting the magnified 

 scale on the drawing. Or you may reverse the process — and it is con- 

 venient to do so — by very carefully drawing the divisions of the mag- 

 nified scale on an ink-ruled straight line on a card, which card you 

 keep, noting on it the objective and eye-piece used, and the exact tube- 

 length employed, in the delineation of your magnified scale. Using 

 the same eve-piece and objective, and the same tube-length, you project 

 the object itself on the card, and you can thus measure oft'the dimensions 

 of its various parts with perfect accuracy on your ink-drawn scale. It 

 your micrometer is ruled with a diagonal you can measure fractional 

 parts, though this can always be determined with accuracy without a 

 diagonal, e. £:, if the diameter of a diatom is one-half of the i.oooth 

 of an inch it is equal to 1-2,000," and so on. 



These remarks lead appropriately to the consideration of the third 

 branch of my subject — how to ascertain the magnifying power of your 

 microscope, for you must do this for yourself, and not accept off' hand 

 the tables of magnification given in the catalogues. To set about it, 

 sketch the ^ J,-,, inch, or take your compasses and measure its image as 

 you see it projected on paper, and then lay off the space so measured 

 on a foot rule divided to a scale of toiths of an inch. If you find the 

 ^i__" thus magnified is exactly = i inch, you microscope is obviously 

 magnifying 100 diameters, because the ^^^^ of an inch is contained 100 

 times in one inch ; if it is equal to 3 inches your microscope is magni- 

 fying 200 diameters ; if to y^" it is magnifying So diameters ; if to 4" it 

 is magnifying 60 diameters. Briefly, the number of times the fraction 

 of an inch, or metre, goes into the greatest diameter of the magnified 

 image is equal to the number of diameters the microscope magnifies. If 

 vou wish to determine the power of your objective alone. I would refer 

 you to the formula given in the last edition of Davis's Practical Mi- 

 croscopy. 



The distance at which you place your eye-piece from the paper when 

 ascertaining the magnifying power of a microscope, or in drawing ob- 

 jects, is sufficiently important to warrant a detailed notice. Three or 

 four who have spoken to me on this subject have maintained that the 

 distance should be a variable one, some urging that (i) the paper should 

 be as far from the eye-piece as the eye-piece is fron'v the stage ; while 

 others consider it should be (2) at such a distance as will secure the 

 projection of a field exactly equal to the visual field, or (3) of an image 

 exactlv equal in apparent size to the image of the object as seen when 

 looked at t/irou<j;/i the microscope. To those who adopt the first view, 

 which is practically identical with the third. I would point out that the 

 distance between the eye-glass and the stage varies with every change 

 in the eye-piece or in the tube-length ; that it is not identical in the case 



