222 USE OF THE MICROSCOPE. 



revolutions, as the teeth commence from the fixed cobweb. 

 In those microscopes provided with a draw-tube, the divisions 

 may always be brought out even numbers, but such wiU not 

 be the case with aU the object-glasses in a microscope not so 

 provided, and the determination of the value of each revolution 

 will then become a rather more complicated matter, as will 

 presently be shown. The value of each revolution of the 

 screw, with the different object-glasses, should be set down in 

 a tabular form, as was shown in the case of the eye-piece 

 micrometers. 



Directions for the Use of the Eye-piece Micrometer. — If there 

 be a draw-tube to the compound body, it should be adjusted 

 according to the table shown at page 220, the object having 

 been brought into the centre of the field, and the micrometer 

 properly adjusted, so that the horizontal fine be in the direc- 

 tion of the diameter to be measured. Read the measurement 

 in the small divisions, and suppose that, ,with the half-inch 

 object-glass, an object occupies seventeen of these ; and it 

 having been shown by the table at page 200, that the value of 

 each division of the eye-piece micrometer, with the half-inch 

 object-glass, was the jjVo ^^ ^^ inch, this number, or rather, 

 the denominator, must therefore be divided by seventeen, 

 and the result will be the ^-i^ oF an inch, or the diameter 

 required to be found; or, should it be preferred to set down the 

 diameter in decimals, then, by adding ciphers to the seventeen, 

 and making it the dividend, and 2,500 the divisor, it may be 

 shown that the diameter is .0068. The positive eye-piece 

 micrometer supplied by Mr. Ross is used precisely in the same 

 way as the above-mentioned instrument ; but, there being no 

 draw-tube, the value of the numbers of the glass micrometer 

 cannot be altered in any way from those mentioned in the 

 table. The squares must be counted as the straight lines are 

 in Mr. Jackson's form, and the dimensions of any object 

 ascertained precisely in the same manner as before described, 

 viz., by dividing the value of each square, as given in the 

 table, by the number of squares occupied by the object ; or, 

 if the decimal notation be preferred, by adding ciphers to the 



