864 



SCIENCE. 



[N. S. Vol. V. No. 127. 



to the object-holder through a triangular 

 bar, moving smoothly but firmly in a tri- 

 angular channel, all lateral motion being 

 eliminated by means of a wedge. 



"The construction of this part is exactly 

 the same as that used in the fine adjust- 

 ment of the microscope. (The triangular 

 bar, wedge and bearing are shown in sec- 

 tion in the figure.) 



" The micrometer screw is provided at its 

 lower extremity with two metal discs in 

 contact, each having one hundred serra- 

 tions, the acute angles of the upper disc 

 pointing to the right, and those of the 

 lower to the left. The discs are revolved 

 by means of the lever A, which is pivoted 

 loosely on the axis of the micrometer screw, 

 and which is provided with a pawl, F. F 

 is actuated by the small lever shown on the 

 upper surface of A, so that when the actu- 

 ating lever is thown to the left, F engaged 

 the teeth of the upper disc, and motion of 

 A to the right elevates the object. The 

 spring rachet, E, prevents any backward 

 motion of the screw head and may be dis- 

 engaged by means of the thumb screw 

 shown. 



" After the screw has been fed up to its 

 greatest extent it is quickly returned by 

 moving the actuating lever to the right, 

 when a small pawl, not shown in the figure 

 because beneath the lever, engages the 

 teeth of the lower disc, and motion of A to 

 the left depresses the object carrier. The 

 amount of elevation of the object is con- 

 controlled in an entirely automatic manner. 



" The stop I is a rigid attachment and is 

 provided with an index, H. C is a gradu- 

 ated disc pivoted around the axis of the feed 

 and movable by means of the lever, B. Two 

 graduations on C correspond to one of the 

 notches on the disc of the micrometer 

 screw. 



" Beginning at zero of the scale, the cir- 

 cumference of C has an inclined plane for 

 the following purpose : 



" The disc C being set so that the number 

 indicating the desired thickness of section 

 is opposite the index, the stop post of A is 

 held against the stationary stop, I, by the 

 long spring ; F engages a tooth of the upper 

 disc. If now the lever A is moved to the 

 right, F will continue to engage the tooth 

 of the disc until the guide post, L, comes in 

 contact with the inclined plane on the mar- 

 gin of C, which disengages the pawl, F, 

 from the tooth of the disc exactly at the 

 zero point of the scale. It will thus be seen 

 that no matter how far A may be moved, 

 F will act only through a certain prescribed 

 distance, governed bj' the position of the 

 disc C, and that the amount of elevation of 

 the object is definitely indicated by the 

 position of H on the scale. 



" The operation of the feed arrangement 

 may be made either automatically or by a 

 separate movement of A by the hand ; the 

 former for serial and ordinary sectioning ; 

 the latter for very thick sections or for sec- 

 tions of irregular thickness. 



" If automatic feed is desired, the sliding 

 stop on the base of the microtome frame 

 (see figure) is adjusted to act as a stop for 

 A, carrying it through the required arc, 

 when the sliding carriage is moved forward 

 and backward for cutting the section. 



" When the hand feed is used, the adjust- 

 able stop may be slid back out of the way, 

 or removed from the instrument altogether. 



" The advantages of this feed may be 

 summed up, as follows : 



" Simplicity. 



" Great accuracy. 



" Short feed (distance object is elevated by 

 movement of one tooth of the disc, being 2 

 microns) . 



" The feed entirely independent of 

 amount of motion given to feed lever, A. 



" Convenience of setting for any desired 

 thickness. 



" Automatic or hand feed at will. 



"Object elevated when object carrier is 



