the blood, and, consequently, over-compression of the corpuscles, and will thereby greatly facilitate examination.
In making these preparations care should be taken to sterilize the needle employed, otherwise grave accidents might occur. It is, of course, unnecessary to sterilize the slides and cover-glasses.
Characteristics of a successful preparation.—On holding a successful preparation up to the light, one or more areas, each made up of three zones, the different zones shading into each other, can be made out by the naked eye. Each such area includes a peripheral zone of a reddish tinge, a middle zone having a somewhat iridescent look, and a central zone absolutely devoid of colour. Successful preparations may be recognized by the presence of these zones. Preparations not exhibiting this appearance should be rejected; it is waste of time to examine them.
On examining successful preparations with the microscope it will be found that the central zone or area contains few or no blood-corpuscles. This zone may be designated the "empty zone." Proceeding outwards from this we come on an area occupied by scattered, isolated, compressed, and much-expanded corpuscles—the "zone of scattered corpuscles." Farther out the corpuscles become more numerous and less expressed (Fig. 16). Gradually, as we trace the film still farther outwards, the corpuscles are found approximated to each other, until, finally, the peripheries of the corpuscles are mostly in touch—the "single-layer zone." Farther out the corpuscles, though still lying flat, are found to overlap each other or are piled one on the top of the other—the "zone of heaped-up corpuscles." Beyond this zone the corpuscles are arranged in rouleaux the "zone of rouleaux." At the extreme margin of the preparation the corpuscles tend to break up and run together so as to form a narrow border of free hæmoglobin, the individual corpuscles perhaps being indistinguishable the "zone of free hæmoglobin." Each of these zones should be studied, for each may afford special information about the malaria parasite.
