1890.] MICEOSCOPICAL JOURNAL. 5 



at the focus of the eye-glass. It is ruled like an ordinary measure on 

 a slip of glass, with every loth line long and every 5th line longer 

 than the others, but shorter than the loth lines. The actual size of the 

 divisions is, of course, immaterial, but their value must be determined 

 by means of the stage micrometer. The scale is brought to bear on 

 the object to be measured, by moving the latter into the centre of the 

 field, and rotating the eye-piece till the divisions of the scale lie at 

 right angles to the diameter to be measured. The micrometer is then 

 shifted till one of the longer lines coincides w^ith one end of the object, 

 and the number of divisions between this point and the opposite edge 

 read off. The scale may be made more delicate by the application of 

 the diagonal scale introduced by Hartnack. In this form, the vertical 

 lines are crossed by two parallel lines at a distance from each other of 

 five divisions of the scale, and the parallelogram thus formed is crossed 

 by a diagonal. The lengths of the lower segments of the 50 vertical 

 lines cut off by the diagonal, thus increase progressively from o.i to 

 5.0, so that, when it is desired to obtain an exact measurement of an 

 object between these limits, it is only necessary to find the segment 

 whose length coincides with the diameter to be measured, which it will 

 give in j-ios of the value of the smaller divisions, whatever these may 

 be. Thus, suppose the length of the segment is 2.6. If each of 

 these smaller divisions represent i-iooo", then the length of this seg- 

 ment corresponds to 2.6-1000" or 0.0026". 



Owing to the difficulty of accurately adjusting the lines of the eye- 

 piece micrometer to those of the stage micrometer, or the edges of ob- 

 jects, while observing under high powers, Jackson introduced a form 

 in which this difficulty is to a great extent obviated. It consists of the 

 same scale as the last described, but the ruled glass is set in a brass frame 

 in which it can slide endways, the motion being communicated by 

 means of a screw working against one end of the glass, while the other 

 end is supported by a spring. 



The brass frame being inserted into the eye-piece through the slits, 

 and roughly adjusted by hand, the final adjustment can be readily and 

 quickly made by working the scale backwards or forwards by means of 

 the screw till it is in the required position. 



Ramsden's micrometer is a more complicated but at the same time 

 more accurate instrument. It consists essentially of two fine parallel 

 wires or spider threads, stretched across the field of the eye-piece, one of 

 which can be caused to approach and recede from the other by means 

 of a micrometer screw with a graduated head., 



A portion of the field of view on one side is cut off at right angles 

 to the filaments, by a scale formed of a thin plate of brass, notched 

 along the edge. Each notch is equivalent to one complete turn of the 

 screw, and every fifth notch is made deeper than the others, so that the 

 number of complete revolutions of the screw can be read oft^ without 

 taking the eye from the object. 



The value of the divisions of this micrometer must, of course, be 

 determined, as usual, by means of an accurately ruled stage microme- 

 ter. The object to be measured is brought into such a position that one 

 of its edges coincides with the stationary filament ; the other thread is 

 then moved by means of the screw until it appears to lie in contact 

 with the other edge of the object. The number of notches on the scale 



