TOL. XIX.] PHILOSOPHICAL TRANSACTIONS. Qg 



N° 213,) he saw insects in water, so small, that 30000 could not more than 

 equal a coarse sand; but I believe it will seem a paradox to him, when told that 

 they can be seen only by applying the bare eye to a portion of water wherein 

 they are contained. 



I have examined many transparent fluids, as water, wine, brandy, vinegar, 

 beer, spittle, urine, &c. and do not remember to have found any of these, with- 

 out more or less of the bodies of these insects; but I have not seen any in mo- 

 tion, except in common water, that has stood for some time, as has been ob- 

 served by Mr. Leuwenhoeck ; though I do not remember he has observed that 

 they are existent in the water before they revive. In the river, after the water 

 has been thickened by rain, there are such infinite numbers of them, that the 

 water seems in great part to owe its opacity and whiteness to these globules. 

 Rain-water, as soon as fallen, has many, and snow-water has more of these 

 globules : the dew that stands on glass windows has them ; and forasmuch as 

 rains and dews are continually ascending or descending, I believe we may say 

 the air is full of them ; they seem to be of the same specific gravity with the 

 water they swim in, the dead remaining in all parts of the water ; of many 

 thousands that I have seen, I could discern no sensible difference in their dia- 

 meters, appearing of equal size in water that has been boiled; they retain their 

 shapes, and will sometimes revive. 



There is another sort of insects which I have this way seen ; but these are 

 not so frequently (at least this winter season) to be found; they are much longer 

 than the former; they can transform themselves into many shapes; they are 

 for the most part elliptical, but sometimes they contract themselves so as to be 

 almost globular; and sometimes they extend themselves, so as to be twice or 

 three times longer than broad ; these sometimes turn themselves round on their 

 axes and diameters as they go; they consist of transparent and opaque parts. 



The first of these natural microscopes performs its effects by the same laws, 

 viz. by the refraction of the rays of light, as the glass ones, and differs from 

 them in nothing but its material, water : but vvhen I began to attempt to satisfy 

 myself how objects are distinctly seen in a spherule of water, I found it at first 

 somewhat difficult to explain ; for whereas objects being placed in the focus of 

 a convex glass, and consequently of water, are seen distinctly to the eye on the 

 other side the glass, and so the reason of the former is obvious enough; but it is 

 as certain, that if an object be placed so much nearer to the eye than the focus 

 of a sphere, as to be within its surface, the rays of light must come too much 

 diverging, to show the objects they come from distinctly. 



But at length, that other known property, if I may so call it, of light falling 

 on diflferent mediums, coming into my thought, viz. reflection, I found there 



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