1886.] 



MICKOSCOPICAL JOITRlSrAL. 



139 



screw may push or pull the web, but it 

 cannot do both accurately. When meas- 

 uring, the web should always be moved in 

 the same direction ; when set for the next 

 measurement it should be returned past 

 the starting point, and then brought 

 carefully to that point again , so the traverse 

 of the web shall be continuously in one 

 direction from point to point. This 

 applies to all screw measurements, and 

 there are mechanical reasons for a strict 

 observance of this rule: — A properly 

 constructed micrometer head should be 

 always divided and numbered, and the 

 revolution, when measuring, should follow 

 the run of the numbers. 



To-day I again tested a newly ruled 

 plate with the screw. When the milled 

 head was revolved in the proper direction 

 every space of looth, and also of loooth 

 of an inch, registered exactly on the 

 micrometer head. A reversal of the 

 operation produced an error of ^f^^u inch 

 in the whole length of the scales. Several 

 repetitions of this exercise with other parts 

 of the screw produced the same result. . 



A well-cut line on glass shows a narrow 

 black central line between two more or 

 less irregular edges. This black line 

 represents the extreme bottom of the cut. 

 This alone determines the exact position 

 of the line, and not the irregular or 

 illy-defined edges at the surface of the 

 glass. 



The periscopic is the only style of eye- 

 piece I have used for micrometry. The 

 field is wide and flat, and the focus, being 

 below the field lens, allows an easy change 

 of eye-piece magnification without dis- 

 turbing the micrometer adjustments. 1 

 have used the periscopic i, }^ , and 

 >^-inch. The i-inch is probably the most 

 perfect of these eye-pieces; the j/^-inch, 

 with the vertical illuminator, presented 

 a rather dim field, and sharp definition of 

 the edge of the corpuscle could not be 

 obtained. 



The number of corpuscles in my list was 

 not large, but each one was very carefully 

 and deliberately measured. I did not 

 have the time to make a larger number of 

 measurements. 



S. G. Shanks. 



Albany, June 2d, i886. 



MICROSC OPICAL SOCIETIES. 



Washington, D. C. 

 Forty-fifth regular meeting. May 25th, 

 [886. 

 Dr. Theobald Smith, of the Bureau of 



Animal Industry, Department of Agri- 

 culture, addressed the Society, by invita- 

 tion, upon ' A Few Simple Methods of 

 Obtaining Pure Cultures of Bacteria.' An 

 abstract of his remarks is published on 

 another page. 



Dr. Schaeffer said that in his opinion the 

 bacterial origin of disease was not yet 

 proved. His position was one of healthy 

 scepticism. He thought there was a 

 fashion in science as well as in dress, and 

 just now it was the fashion to go to Ger- 

 many for our theories about bacteria. He 

 did not advocate a slavish adherence to 

 or belief in everything written by the 

 authorities. He thought that just now we 

 were at the crest of the wave of bacteriol- 

 ogy, and that we should soon begin the 

 decline. In his opinion the true cause of 

 disease was of a chemical nature. 



Mr. Skinner asked whether Dr. Freire's 

 researches on the bacteria said to be the 

 cause of yellow fever were generally 

 accepted as sound, and also alluded to the 

 theory that bacteria themselves do not 

 cause disease but the ptomaines generated 

 by their presence, or, in other words, that 

 the active cause of disease is not vegetable 

 but chemical. 



Mr. Hitchcock said : — Nowhere is better 

 work in this line being done than in the 

 Bureau of Animal Industry of the Depart- 

 ment of Agriculture. Laborious and 

 patient investigations have been carried 

 on quietly for years and the results are 

 just now beginning to appear. Alluding 

 to Dr. Schaeffer's remarks, he said : — If a 

 germ is isolated, cultivated, and a pure 

 culture obtained which will invariably 

 produce a certain disease in the animal 

 inoculated ; then, if the germ is not the 

 cause of the disease, what is the cause ? 



Dr. Taylor said : — If, after inoculation 

 with the B. iubcir^ilosis, we find tuberculo- 

 sis in any large number of the animals in- 

 oculated, then it is reasonable to sup- 

 pose that the bacillus is the cause of the 

 disease. 



Dr. Seaman stated that the discoveries 

 leading to the development of the germ 

 theory had their origin in researches upon 

 much larger objects than bacteria, viz., 

 mycoderms. 



Dr. Smith closed the discussion by 

 saying that no original work in this line 

 is accepted unless full and detailed ac- 

 counts of all experiments are given, so 

 that the investigator's methods can be 

 exactly followed. So far he had seen no 

 such accounts from Dr. Freire. Much of 

 the scepticism as to the bacterial origin of 

 disease is due to the fact that most of the 



