354 LAY SERMONS, ADDRESSES, AND REVIEWS. [xy. 



But the skill of the microscope-makers of the eigh- 

 teenth century soon reached its limit. A microscope 

 magnifying 400 diameters was a chef cPceuvre of the 

 opticians of that day ; and, at the same time, by no 

 means trustworthy. But a magnifying power of 400 

 diameters, even when definition reaches the exquisite 

 perfection of our modern achromatic lenses, hardly 

 suffices for the mere discernment of the smallest forms 

 of life. A speck, only -^-tli of an inch in diameter, has, 

 at 10 inches from the eye, the same apparent size as an 

 object 10 ^ 00 th of an inch in diameter, when magnified 

 400 times ; but forms of living matter abound, the 

 diameter of which is not more than 40 ^ - 5 -th of an inch. 

 A filtered infusion of hay, allowed to stand for two 

 days, will swarm with living things, among which, any 

 which reaches the diameter of a human red blood-cor- 

 puscle, or about g^ th of an inch, is a giant. It is 

 only by bearing these facts in mind, that we can deal 

 fairly with the remarkable statements and speculations 

 put forward by Buffon and Needham in the middle of 

 the eighteenth century. 



When a portion of any animal or vegetable body 

 is infused in water, it gradually softens and disinte- 

 grates ; and, as it does so, the water is found to swarm 

 with minute active creatures, the so-called Infusorial 

 Animalcules, none of which can be seen, except by the 

 aid of the microscope ; while a large proportion belong 

 to the category of smallest things of which I have 

 spoken, and which must have all looked like mere 

 dots and lines under the ordinary microscopes of the 

 eighteenth century. 



ou dans l'aninial de meme esp&ce qui l'a prodnit." — Kouvelles Observations, 

 p. 169. 



" Les naturalistes ont generalement cru que les animanx mieroscopiques 

 etaient engendres par des ceufs transportes dnns Pair, ou deposes dans des 

 eaux dormantes par des insectes volans." — Ibid., p. 17G. 



