ZOOLOGY AND BOTANY, MICROSCOPY, ETC. »)09 



bearings which provide for taking up lost motion and for eliminating 

 any wear that may occur. At its upper end this feed screw is connected 

 with the object-supporting socket by means of a ball-socket bearing, to 

 which the object-support is firmly held down by a strong steel spiral 

 spring. The automatic feed mechanism consists of an accurately cut 

 ratchet wheel, keyed to the vertical feed screw, in the teeth of which a 

 hardened steel pawl engages. By means of a lever, extending to the 

 graduated scale shown on the front of the main frame base, this pawl 

 may be thrown out of action by turning the index finger to the extreme 

 left, or it may be set to cut sections of any desired thickness. Each 

 division of the graduated scale marks 5 microns. Along, vertical shaft, 

 extending to the lever arm at the top of the main frame and connected 

 at the bottom by a simple lever device, moves the pawl backward and 

 forward simultaneously with the movement of the knife, so that wher- 

 ever the index is placed on the graduated scale, the object is correspond- 

 ingly raised at each swing of the knife. The crank at the bottom can 

 be used independently of this automatic motion and provides for quickly 

 raising or as quickly lowering the object whenever desired. The whole 

 feed mechanism is simple in construction, and, being covered by the 

 extended base of the main frame, is protected from dust and drippings. 

 It may easily be reached from below and it cannot easily get out of order. 

 A convenient drip pan, at the top of the socket in which the object- 

 clamp sets, may be quickly unscrewed and as easily replaced. It is besL 

 to remove it when the freezing chamber is attached. This is heavily 

 nickel-plated, as are all the screws and exposed parts. 



Embedding in Paraffin.* — R. T. Hance says that when embedding 

 very small objects, such as insect larvae or small flowers or anthers, in 

 paraffin it is most convenient to orient them one behind the other. 

 This method allows a single block to be made of three or four to a 

 dozen pieces of tissue, and these may be cut in one ribbon. This 

 obviously eliminates a great deal of the labour in making a block for 

 each separate object, cementing it to the holder, trimming it, and 

 adjusting the microtome each time. In tlie ribbon it is easy to see 

 where one piece of tissue ends and the other begins, as there are usually 

 several blank sections of paraffin between them. It is relatively simple 

 to arange the tissue in line under a carbon bulb with warm needles, but 

 a difficulty is met with when an attempt is made to place the paraffin 

 mould in water for cooling. The material is shaken from position, 

 and must be re-oriented. This has buen overcome in the following way. 

 A watch-glass is used as a mould for embedding small objects, and a 

 Petri dish is convenient for larger tissue. When the tissue is ready to 

 be embedded, the dish is heated to the melting-point of the paraffin 

 under the electric bulb. It is then placed in a crystallization dish, with 

 two slides beneath it to prevent it from touching the bottom of the 

 container. Paraffin is then poured into the small dish, and the objects 

 oriented as desired, the heat of the electric bulb keeping the paraffin 

 melted. Then the light is turned off, and cold water is poured into the 



* Trans. Amer. Micr. Soc, xxxv. (1916) pp. 137-8. 



