MICROSCOPICAL TECHNIQUE 51 



Hanging block. It is frequently desirable to determine 

 the mode of germination of bacterial spores, and the mode 

 and rate of fission of the vegetating cell. This is most con- 

 veniently done by the use of Hill's hanging block. Pour 

 melted nutrient agar into a Petri dish to the depth of one 

 eighth of an inch. Cool the agar. Cut from it with a sterile 

 knife a block one quarter to one third of an inch square, and 

 of the thickness of the agar in the dish. Place the block, 

 under surface down, on a glass slide, and protect from dust. 

 Prepare in sterile water a suspension of the organism to be 

 examined, if it has been grown on a solid medium, or, if a 

 broth culture is used, spread the suspension on the upper sur- 

 face of the block as though making a cover-glass smear. Place 

 the slide with the block in the 37 C. incubator for five or ten 

 minutes to dry. Then lay a clean sterile cover glass on the 

 inoculated surface of the block in close contact with it, avoid- 

 ing air bubbles as far as possible. Remove the block from 

 the slide and invert the same. With a platinum loop run a 

 few drops of melted agar around the edge of the block, to fill 

 the angles between the sides of the block and the glass. This 

 seal prevents slipping of the block. The block should again 

 be placed in the incubator for five to ten minutes to dry. In- 

 vert this preparation over a moist chamber made by cement- 

 ing a ring of glass to an ordinary slide. The hollow ground 

 slide cannot be used on account of the depth of the block. 

 The cover should be sealed in place with white wax or 

 paraffin. Vaseline softens at 37 C. and allows the cover 

 glass to shift. The preparation may now be observed with 

 the -J^-inch immersion objective. 



Measuring bacteria. Measurements under the compound 

 microscope are made by means of an ocular micrometer, a 

 disk of glass which is placed on the diaphragm within the 



