PRACTICAL CONDUCT OF EXERCISE II 135 



these exercises. Advantages that it offers are that the records are more flexible, do not 

 curl, are less brittle, allowing samples to be more readily put in an exercise book, have 

 nearly a matt surface, also that it is far less expensive than mastic vanish. A recipe for it 

 which 1 prefer is slightly modified from that published by Kent {Jnl of Fhysiol. vol. 1, 

 p. xxi, 1916) ; it is as follows : 



' Powered amber resin ', 130 grm. 



Paraffin wax, melting-point 52° C, cut into fine shavings, 10 grm. 



Petrol, first grade, 1 litre. 

 Dissolve in the cold and keep in a cool place, shaking from time to time. The resin will 

 nearly all dissolve, and at the end of seven days the liquor can be decanted off or filtered 

 and is ready for use; earlier than that I find its action uncertain, the soot-layer not being 

 rendered reliably adherent to the paper. In using this fixative the tray in which the 

 varnishing is done should be quite dry before putting the varnish into it, not only from 

 traces of water but also from old varnish. For aim. graphic the fixative is put in a 

 shallow iron tray on the floor and the paper slowly and evenly drawn through it and then 

 drained vertically from all excess. Pinned then vertically, with both sides free to the air, the 

 tracing is dry in a few minutes. I do not find it advisable to dilute with fresh petrol after 

 the varnish has been in use for some time and become thickened. The thickened residue 

 contains a large percentage of heavy oils and is not reliable for use as varnish even w^hen 

 thinned with petrol. But the varnish is so cheap as compared with the usual mastic 

 varnish that the sacrifice of the old residue is of small account. It is, however, more 

 inflammable than mastic varnish. For that and other reasons the longer student tracings 

 are best varnished after the class-meeting and the stock varnish kept under the usual 

 precautions for petrol. This varnish, too, leaves the white of the line traced by the style 

 somewhat less pure than does a first-rate thin mastic varnish : a slight defect of no conse- 

 quence for ordinary class-tracings. 



Graphics as Transparencies for Lantern Projection. 



Effective transparencies for the projecting-lantern can be prepared by cutting the 

 required piece from the fixed and varnished sooted paper, passing it through turpentine, and 

 mounting it in Canada balsam between lantern-slide covers or other suitably-sized glass 

 plates, and binding the edges with lantern-slide binding-strip paper in the usual way. An 

 actual record is thus available as a permanent lantern slide. Glazed white paper for 

 kymograph and similar work is supplied by the makers in various thicknesses ; the thinner 

 kinds yield the best transparencies. A thin paper for long tracings runs closer and more 

 flexibly round the recording-drum. 



Perfusion. — The pail reservoir for hot water for the jacket, and the oxygen cylinder, can 

 of course serve more than one perfusion apparatus at the same time. Text-fig. 6 shows them 

 serving two adjacent tables. If the Ringer-Locke fluid be poured into the reservoirs shortly 

 before use, the employment of an oxygen cylinder supply can be dispensed with, the oxygena- 

 tion by the air when the reservoir is being filled sufficing for four or five subsequent hours. 

 Addition of dextrose is not essential ; class experience obtains excellent results without it. 



As to the rabbit serving as preparation, one's experience indicates for preference an 

 animal about three-quarters grown, not long-haired and not albino. 



