THE MICROSCOPE IN MEDICINE AND SANITATION. 121 



in a frame and provided with screws, by which they can 

 be pressed together, thus reducing the meat to a thin 

 transparent layer. The frame bears on its under side 

 two ridges which fit into grooves in a special corrugated 

 stage of large size on the microscopes designed for this 

 purpose. Under a very low objective, of ij-inch equiva- 

 lent focus, the compressed frame is then examined, being 

 slid along until its whole width has been covered, and then 

 shifted to the next groove, so that a new field may be 

 examined. In this way the whole sample may be viewed 

 in five minutes by a trained observer. 



Under the microscope the pork is seen to be made up 

 of the long cylindrical cross-striated fibres of voluntary 

 muscle, and if Trichina be present the cysts will be seen 

 here and there as spindle-shaped, whitish bodies. In 

 sections of trichinous pork prepared after paraffin im- 

 bedding with the microtome, the worms may be seen 

 coiled up inside the cysts, as shown in Fig. 47. 



9. The Microscopy of Drinking "Water. — The sanitary 

 quaHty of water depends primarily upon the presence 

 or absence of disease germs, generally introduced in 

 sewage. The most important evidence as to its character 

 is therefore obtained by the bacteriological and chemical 

 analyses, which indicate with great delicacy the presence 

 of minute traces of sewage pollution. A microscopical 

 examination may, however, often add information of 

 value by showing the presence of starch-grains, yeast- 

 cells, fragments of vegetable tissue, and certain Infusoria 

 characteristic of decomposing material; organisms pecul- 

 iar to a particular pond or stream have sometimes proved 



