120 THE AMERICAN MONTHLY [Api' 



nntely, however, if with any compound objective we re- 

 peat t.his operation several times, merely varying the dis- 

 tances, we obtai?i as many dif event values of f as there 

 are distances used, instead of obtaining but one value for 

 all distances as we do with a single lens. Mr. Cross has 

 already pointed out this circumstance, which results from 

 the fact that the modern achromatic objective has con- 

 siderable thickness from its anterior to its posterior sur- 

 faces, and that it has, properly speaking, no true optical 

 centre.'' 



Dr. Woodward, in his paper, gives several examples of 

 ascertained discrepancies while using the formula in 

 achromatic object-glasses, when trials were made with 

 them at varying distances to ascertain their magnify- 

 ing power. 



» ■ » ^ « ■ < 



Making and Staining Cover-Glass Preparations, and For- 

 mulae for Staining Solutions. 



V. A. MOORE. 

 [From his New Book]. 



Make two cover-glass preparations from each of the 

 cultures made in agar, bouillon.and gelatin, and stain one 

 with alkaline methylene-blue and the other with carbol 

 fuchsin. Describe the appearance of the bacteria and 

 make a drawing of a few individual bacteria from the 

 preparations made from the agar culture. Preserve a 

 cover-glass preparation mounted in balsam and label it. 

 Prepare the staining fluids. 



Making Cover-glass Preparations. — (1) From bouillon 

 cultures. Place 2 clean cover-glasses on the tray. With 

 the loop remove a drop of the bouillon culture and spread 

 it in a thin layer over about 2-3 of the surface of the 

 cover-glasses. One loop-full will ordinarily make from 2 

 to 4 preparations. Allow the liquid to dry on the cover- 

 o-lasses in the air. When dry, fix the bacteria to the cover- 



