144 RECENT RESEARCHES 



Others on this subject, is the vast differences of opinion which we 

 find existing among different observers. Professor A. says one 

 thing, Professor B. exactly the opposite. C. gives a full, true, and 

 particular account of what D. immediately contradicts. One chief 

 cause of these differences of opinion is the extremely minute size 

 of the organisms under consideration, rendering the use of very high 

 microscopic powers necessary, and requiring therefore extreme and 

 unusual skill in microscopic manipulation, and causing numberless 

 difficulties in their examination, and almost infinite sources of error, 

 which can only be overcome by the highest masters of precise 

 experiment. And as we often have no opportunity of forming an 

 opinion of the skill of the observers, it is frequently difficult to 

 arrive at any definite conclusion, not knowing whom to believe. 

 But when we find that experiments are well and carefully devised, 

 and described with precision (by such men as Pasteur and Tyndall) 

 we cannot fail to attach great weight to their conclusions. So when 

 well-known and accomplished microscopists, (as Cohn, Koch, 

 Dallinger, etc.,) give us the results of their observations, we cannot 

 doubt the reliability of their assertions. 



With reference to the difficulty of microscopical investigation, 

 Cohn remarks that " so long as the makers of microscopes do not 

 place at our disposal much higher powers, we shall find ourselves, 

 as regards the domain of the Bacteria^ in the situation of a trav- 

 eller who wanders in an unknown country at the hour of twilight, 

 at the moment when the light of day no longer suffices to enable 

 him clearly to distinguish objects, and when he is conscious that, 

 notwithstanding all his precautions, he is Hable to lose his way." 

 My object in the present paper will be to try and sift out, and 

 lay before our readers, some of the facts that have been clearly 

 established, and then to see what conclusions may be fairly deduced 

 from them. 



The Bacteria were known as early as 1675 ^o Leeuwenhock, 

 who may be called " the father of Microscopy," but for a long time 

 they were looked upon as mere scientific curiosities, and it has only 

 been, of late years, by long and patient research, that their history 

 has been made out, and that they have been relegated to their 

 proper position in the organic world, and it has been known how 

 important a role they play in the economy of nature. Ehrenberg 



