586 



LABORATORY EXERCISES 



used. The step micrometer has loo intervals distinctly indicated in the middle. 

 It is necessary to find the number of intervals of the object micrometer covered 

 by loo intervals of the step micrometer, viz., with objective 3 (16 mm.), at a tube 

 length of 141 mm., 100 intervals of the step micrometer cover 100 intervals of the 

 object micrometer, equal to i mm. 



One interval of the step micrometer is as i : 100 = o.oi or 10 micra. Micrometer 

 value = 10. • 



With objective 6 (4 mm.) at a tube length of 160 mm. 100 intervals of the 

 step micrometer cover 20 intervals of the object micrometer = 

 0.2 mm. One interval of the step micrometer therefore 0.2 = 100 

 = 0.002 or 2 micra. Micrometer value 2. 



This new micrometer eliminates the time-consuming measure- 

 ment with three or more figures after the old method and is still 

 more accurate. 



Comment.- — M. Nobert of Griefswald in Prussia engraved lines 

 more than 100,000 to the space of an inch. 



Laboratory Work. — Compute the various micrometric values 

 according to the three methods outlined above. After determin- 

 ing these values for the various combinations of which your 

 microscope is capable measure the following objects: 



Spores of black mould, spores of slime moulds studied, various 

 diatoms, etc. Practice these methods until you have perfected 

 yourself in them. 



REFERENCES 



Fig. 208. — 

 Scale of step 

 micrometer. 



Beale, Lionel S.: How to Work with the Microscope, 1868 (4th 

 Edition), pp. 35-38. 



Behrens, Julius W., trans, by Rev. A. B. Hervey: The 

 Microscope in Botany. A Guide for the Microscopical 

 Investigation of Vegetable Substances, Boston, 1885, pp. 

 120-133. 



DoLLEY, Charles S.: Notes on the Methods Employed in Biolog- 

 ical Studies, 1889, pp. 18-20. 



Gage, Simon Henry: The Microscope. An Investigation of Micro- 

 scopic Methods and of Histology, 1899, pp. 100-108. 



LESSON 2 



Directions for Plugging Test-tubes and Flasks. — Before sterilization all test-tubes 

 and flasks must be carefully plugged with cotton-wool, and for this purpose best 

 absorbent cotton-wool (preferably that put up in cylindric one-pound cartons and 

 interleaved with tissue paper) can be used (Fig. 209). 



I. For a test-tube or a small flask, tear off a piece of cotton-wool some 10 cm. 

 ong by 2 cm. wide from the roll. 



