The Microscope. 281 



it is necessary to interrupt the beam of light regularly, for con- 

 tinuous observation, the effect of intermittent light may be ex- 

 hibited to some extent by an apertured disk like that above de- 

 scribed, twirled by the thumb and finger or revolved like a top 

 by means of a string ; or by using a larger apertured disk fitted 

 to a rotator, and placed between the source of light and the 

 mirror of the microscope. — Scientific American. 



DR. JOHNSON ON MICROBES. 

 All anticipation of future knowledge and suggestions which 

 those who come after find to be correct, have a peculiar kind of 

 interest and are worth noting, even though the scientific basis 

 on which they are founded may be questionable. There is an 

 observation of Samuel Johnson's on the subject of dysentery 

 which fairly illustrates this. In a letter to Mrs. Thrale, dated 



November 12, 1781, he says : "If Mr. will drink a great 



deal of water, the acrimony that corrodes his bowels will be 

 diluted, if the cause be only acrimony ; but I suspect dysenter- 

 ies to be produced by animalcule, which I know not how to 

 kill." Johnson used the word " animalcule ;" a modern writer 

 would say " microbes." — N. Y. Med. Record, 1885. 



THE MICROSCOPE FOR OCTOBER. 



The leading article in the Microscope for October is from 

 the pen of Mr. Walmsley, of Philadelphia, and is the first of a 

 series on " photo-micrographs," and, it is safe to say that all who 

 have the pleasure of knowing the author, and of having seen 

 his superb work in the line on which he writes, will read the 

 communication very closely. It is admitted on all sides that 

 Mr. Walmsley has no superior in this department of microscopi- 

 cal technology. Professor Marshal D. Ewell, of the Union Col- 

 lege of Law, Chicago, contributes a well written description of 

 the ruling machine of Professor Rogers, of Harvard Univer- 

 sity, and of the method employed by that great American ex- 

 pert in ruling standard micrometers. Dr. Allen Y. Moore, of 

 Cleveland, gives the result of a critical study of the work of a 

 1-18 oil immersion objective of Zeiss, as compared with a Spen- 

 cer 1-10, and as was to be expected by all who have had any 



