M THE AMERICAN MONTHLY [February, 



The public mind catches up a deal of science now and then, but it is not 

 always very sound. Perhaps there is no doctrine more generally accepted 

 among well-informed people than that of the germ origin of disease, and there 

 is nothing that is dreaded more in city houses than ' sewer gas,' which is 

 supposed to carry all sorts of germs right into the house. Now, while not 

 advocating the general admission of sewer gas into dwellings as a sanitary 

 measure, it does not appear to be quite so pestilential as is generally supposed. 

 Indeed, the number of germs actually found in sewer gas is far smaller than 

 might be expected, and, indeed, it would seem that the conditions prevailing 

 in the sewers are almost inimical to the rapid development of germs. At all 

 events the air of sewers is probably harmful more because it vitiates the air, 

 and in this way lowers the general tone of the human organism, thus render- 

 ing it more easily attacked by disease, than because of the few specific poisons 

 it may carry. 



The microscope in the study of bacteriology.* 



By Dr. THEOBALD SMITH, 



BUREAU OF ANIMAL INDUSTKY. 



A magnification of from 500 to 800 diameters is the most useful in the 

 study of pathogenic forms. The use of illuminators with special reference 

 to that designed by Abbe and now universallv employed in bacteriological 

 work was described. Unstained specimens are examined with a small dia- 

 phragm to bring out the structin-al details. Stained specimens are best ex- 

 amined without a diaphragm. The whole pencil of light is thus utilized. 

 The removal of the diaphragm ohliterates the structure-picture and substitutes 

 the color-picture. The interpretation of the appearance presented by the 

 same object, a spore-bearing bacillus, stained and unstained, under the two 

 conditions, was given in illustration. The advantage of wide-angled lenses 

 in the searching of sections of tissues for bacteria was dwelt upon. The 

 very thin layer of tissue brought into focus at the same time, even in thick 

 sections, eliminates many disturbing elements. 



A description was given of the devices employed in studying bacteria in 

 liquids — and the various cells which are used. The usual way is to examine 

 minute drops of liquid suspended from the under side of a cover glass which 

 rests over a cell in the slide. The great advantages of the so-called stained 

 cover glass preparations in quicklv determining the relative number and kind 

 of bacteria in tissues were mentioned, and the method of preparing described. 

 Special culture cells, such as those of Brefeld, Prazmowski and others are 

 used to elucidate problems of a certain character in the biology of micro-or- 

 ganisms. 



In endeavoring to fulfil the three conditions laid down by Koch, necessary to 

 the complete demonstration of the etiologv of infectious diseases, the micro- 

 scopical examination of tissues for the presence of bacteria comes in first. 

 Histological methods are also of great service in determining the action of 

 bacteria on tissue elements. They inform us whether bacteria are intracellu- 

 lar or not, and what the nature of the pathological process is which is caused 

 by their presence. Before the technique of staining came to its present per- 

 fection, acetic acid, alkalies, ether, and chloroform were frequently employed. 

 Sections of tissues were immersed in strong acetic acid or dilute caustic potash 

 or soda to dissolve the cell protoplasm and the formed elements. The bac- 

 teria remain unchanged and could be easily detected. These methods are 

 crude and rarely used except as accessories to staining methods. The speaker 



*Report of a paper read before the Microscopical Society of Washington, D. C, December 13, 1887. 



