Chap. 6.] Spherical refrafting Surfaces, &c. 235 



qm> and the place of the fpectator O. He will fee only 

 part of the object correfponding to n m, and through 

 the part of the furface A s } and the object will appear 

 to him inverted. 



If the eye is placed nearer the furface than the image 

 is, the object will appear confufed ; for the rays ftrik- 

 ing the eye will then be converging to a place behind 

 it : but as I have been rather prolix on this fubject in 

 the cafe of objects before reflecting mirrors, it will not 

 be neceflary to purfue it farther, as on the fame prin- 

 ciples the reader will with eafe determine the part of 

 an object feen in any medium, bounded by a fpherical 

 furface, the part of the furface through which it is feen, 

 and the rays by which it is feen. The truth of fome 

 of thefe principles may be experimentally fhewn by 

 objects placed in glafs. vefiels, with concave or convex 

 furfaces, filled with water, but as we cannot form a 

 medium rarer than that in which we live, the cafes of 

 objects placed in a rarer rrtedium muft remain efta- 

 blifhed on the fixed bafis of mathematical truth here 

 laid down. If we were indeed to rarify the air in a 

 hollow glafs globe, we might obferve, perhaps, the 

 changes made in the apparent places of an object, ac- 

 cording to the fucceffive degrees of rarefaction ; but 

 ftill the object would be feen through one medium 

 much denfer than the furrounding atmofphere, and be- 

 fore we can examine the apparent fituation of an ob- 

 ject placed in one medium, which is feparated from 

 another by a medium of a different denfity from either, 

 and bounded by a concave and convex furface, we muft 

 endeavour to account for the appearances which are 

 daily before our eyes, namely, the changes made in the 

 apparent places of objects by the interpofition of a denfe 

 /ubftance bounded by fpherical iurfaces, 



In 



