FUNDAMENTAL CONSIDERATIONS 19 



and the top of the celloidin block must be constantly kept wet with 

 80 per cent, alcohol. 



The object is placed in the object carrier and clamped in. By 

 means of the graduated disk at the base of the feed mechanism the 

 thickness, in terms of microns, is regulated after each stroke of the 

 razor. 



Rotary Microtome. When paraffin ribbons are desired, especially 

 for the study of serial sections of material, the rotary microtome 

 surpasses by far the efficiency of the sliding type of instrument. The 

 Spencer Rotary Microtome No. 820 is shown in Fig. 10 and its 

 plan of construction illustrated in Fig. n. In this instrument the 

 sliding part which carries the object clamp (SP) is carried up and 

 down by the block (B). The feed mechanism consists of a rigid 

 bearirfg, on which the feed block (FB) (of which the projection P is 

 a part), is moved by the feed screw (FS). As this block travels to- 

 ward the side on which the balance wheel (W) is located, the sliding 

 part (SP) is forced forward towards the knife one-half as much. The 

 polished surface set against the point (P) is arranged at the proper 

 angle to accomplish this end. The screw, cut with two threads to 

 the millimeter, is revolved by a ratchet feed wheel with 250 teeth. 

 Each tooth represents a forward movement of the object of one mi- 

 cron. The feed can be set for sections from i micron to 60 microns 

 thick, by turning the button at the back of the case until the number, 

 representing the desired thickness, appears opposite the indicator at 

 the small opening in the side of the case near the balance wheel. 

 The total excursion of the feed is 37 mm. This allows a sufficient 

 range for cutting a complete series of sections of a large object with- 

 out the necessity of a break due to resetting the knife and feed me- 

 chanism. The object, after being placed in the object clamp, may 

 be oriented to any desired angle. The clamp is held at its upper 

 limit for orienting or trimming the block by pushing in the pin (F.) 

 The whole knife support may readily be adjusted to and from the 

 object, and is readily clamped in any location by a lever connected 

 with an eccentric cam. The knife is fastened by two clamps and 

 may be turned to any desired angle. The clamps can also be moved 

 toward each other to bring them as near to the ribbon as desired to 

 gain additional rigidity. The groove in the balance wheel is de- 



