Caldwell's automatic microtome. 651 



brass arm : upwards, if it is moving too fast, downwards if too 

 slow. A frequent cause of failure in the proper movement of 

 the band is that the ebonite roller at the bottom of it is 

 allowed to press against the razor. This must be avoided. 



Varying the thickness of the sections. — In fig. 2 will 

 be seen a milled head f, which when turned controls the move- 

 ment of the clicks which acting upon the ratchet-wheel attached 

 to the micrometer screw regulate the thickness of the sections. 

 This may be done so as to allow the clicks to engage one half, 

 one, or several teeth of the ratchet-wheel as may be required. 

 When arranged for one half tooth, the sections will be -po-L__th 

 of an inch (•0025 mm.) in thickness, when arranged to engage 

 a whole tooth ^^^^th of an inch (-005 mm.), and so on. At 

 first it is well to use a whole tooth, as when thinner sections 

 are cut so much depends on the sharpness of the razor. After 

 cutting for some time, the machine will suddenly stop, the 

 object ceasing to rise when the handle is turned. This means 

 that the full extent of the micrometer screw has been reached. 

 It is necessary then to turn the large milled head (fig. 2, c) 

 downwards, which will allow the carriage containing the object 

 to fall to its lowest limit. It will be necessary now to raise 

 the socket (fig. 2, g) in which the object is held so as to be in 

 position to come in contact with the razor. This milled head 

 (fig. 2, c) is useful for rapidly getting the object in proper 

 position and avoiding considerable loss of time in turning the 

 handle. The frame (fig. 2, h), which holds the socket, is 

 arranged with two quadrants, so that the socket may be set at 

 any angle desired, and may be clamped with the milled head 

 underneath it. This is for use when the object has not been 

 symmetrically embedded. The nut (fig. ] , i) is for tightening 

 up the spring which draws the carriage of the machine back 

 after having been pulled forward. In case this does not work 

 properly, it is only necessary to unloose the two screws and, 

 with some strong but blunt pieces of steel placed in the two 

 holes, to rotate the nut so as to give a proper tension to the 

 spiral spring. When this is done, the screws should be tightened 

 up again to keep the nut in place. 



