iL. SEMINAL VERMICULI. 3 
window oppofite a white wall moderately illuminat- 
ed: as, for example, when expofed toa part of the 
fky covered with white clouds. Secondly, the finer 
and thinner the fliders are, on which the drop of 
femen is depofited, the eafier are the tails difcover- 
ed: I prefer talc to chryftal. Thirdly, the drop 
fhould be as thin as poffible, otherwife the origin 
of the tail will not be difcerned; the reft is 
concealed in the liquid; and when the feminal 
fluid is turbid, it is neceffary to dilute it with a 
clearer portion. Fourthly, when the vermiculi 
fwim, as the tailis always a little lower than the 
- body, we muft deprefs the focus of the lens to 
obferve it. Finally, a microfcope of a fingle 
lens, fuch as that called Leeuwenhoeck’s, fhould 
pofitively be preferred to the compound micro- 
fcepe. 
Although M. de Buffon, in his obfervations, 
mentions only one or two of the precautions allud- 
ed to, and which he feems to have ufed, Iam un- 
willing to think he negletted the reft ; for the ex- 
ception of only one would prevent him from mak- 
ing the real obfervation. He fays he always ufed 
the compound microfcope in examining the femen | 
of man and animals. I doubt not that his micro- 
{cope was as excellent as he affirms; but it was 
a compound one, and had all the defects of com- 
pound microfcopes, among which is fpecially plac- 
ed, the object never being feen fo diftiné, or its 
A2 outlines 
