H. G. Wells 
The Stolen Bacillus [EPUB ebook] 

‘This again, ‘ said the Bacteriologist, slipping a glass slide under the microscope, ‘is a preparation of the celebrated Bacillus of cholera-the cholera germ.’
The pale-faced man peered down the microscope. He was evidently not accustomed to that kind of thing, and held a limp white hand over his disengaged eye. ‘I see very little, ‘ he said.
‘Touch this screw, ‘ said the Bacteriologist; ‘perhaps the microscope is out of focus for you. Eyes vary so much. Just the fraction of a turn this way or that.’
‘Ah! now I see, ‘ said the visitor. ‘Not so very much to see after all. Little streaks and shreds of pink. And yet those little particles, those mere atomies, might multiply and devastate a city! Wonderful!’