94 APPENDAGES TO THE MICROSCOPE. 



fine-threaded screw, the head of which is divided at its edge into 

 a convenient number of parts, which successively pass by an index 

 as the milled-head is turned. A portion of the field of view on 

 one side is cut off at right angles to the cobweb-threads, by a scale 

 formed of a thin plate of brass having notches at its edge, whose 

 distance corresponds to that of the threads of the screw, every 

 fifth notch being made deeper than the rest for the sake of ready 

 enumeration. The object being brought into such a position that 

 one of its edges seems to touch the stationary thread, the other 

 thread is moved by the micrometer-screw until it appears to lie in 

 contact with the other edge of the object ; the number of entire 

 divisions on the scale shows how many complete turns of the screw 

 must have been made in thus separating the threads, while the 

 number to which the index points on the milled-head shows what 

 fraction of a turn may have been made in addition. It is usual, 

 by employing a screw of 100 threads to the inch, to give to each 

 division of the scale the value of l-100th of an inch, and to divide 

 the milled-head into 100 parts ; but the absolute value of the 

 divisions is of little consequence, since their micrometric value 

 depends upon the Objective with which the instrument may be 

 employed. This must be determined by means of a ruled slip of 

 glass laid upon the stage ; and as the distance of the divisions 

 even in the best-ruled slip is by no means uniform,* it is advisable 

 to take an average of several measurements, both upon different 

 slips, and upon different parts of the same slip. Here the Draw- 

 tube will be of essential use, in enabling the Microscopist to bring 

 the value of the divisions of his Micrometer to even numbers. 

 Thus, suppose that with a l-4th-inch Objective, the tube being 

 pushed in, a separation of the lines by one entire turn and 

 37-100ths of another were needed to take in the space between 

 two lines on the ruled slip whose actual distance is one l-1000th of 

 an inch, then it is obvious that 137 divisions on the milled-head 

 are equivalent with that power to a dimension of 1-1 000th of 

 an inch, or the value of each division is 1-137, 000th of an inch. 

 But as this is an awkward number for calculation, the magnifying 

 power may be readily increased by means of the Draw-tube, until 

 the space of l-1000th of an inch shall be represented by a separa- 

 tion of the cobweb-threads to the extent of 150 divisions ; thus 

 giving to each division the much more convenient value of 

 1-150, 000th of an inch. The Microscopist who applies himself 

 to researches requiring micrometric measurement, should deter- 

 mine the value of his Micrometer with each of the Objectives he is 

 likely to use for the purpose ; and should keep a table of these 

 determinations, recording in each case the extent to which the 



* Of the degree of this inequality, some idea may be formed from the 

 statement of Hannover, that the value of the different divisions of a 

 glass ruled by Chevalier to l-100th of a millimetre, varied between the 

 extreme ratios of 31 : 36, the mean of all being 34. 



