Chap. 6.] Real and apparent Places of Objeffs. 229 



diverged from Qj but from a point near to or farther 

 from him, according as the medium, in which he is, is 

 denfer or rarer than the medium in which the point 

 of diverging rays is fituated. Let QJ be an incident 

 ray proceeding after refraction in the direction I M, 

 cutting QJ) a perpendicular to the furface A B in q, 

 q will be the point from which the rays appear to di- 

 verge. In the triangle QJ q y QJ \\q\\ fine ofl^O 

 : fines of I QJ3 ; that is, fmce QJ3 is parallel to I P 

 :: fine of refra6Uon : fine of incidence. If I is very 

 near to O, the lines QJ, ql, will be very nearly equal 

 to QJD and q O, and a perfon being placed in the di- 

 rection QJD produced will conceive that the rays di- 

 verged from q, when QP : qQ :: fine of refraction : 

 fine of incidence. 



Upon this principle we can find the actual fituation 

 of any object feen in a medium different from that in 

 which we are, or feen through different mediums. 

 Let Q^R (Fig. 13, 14.) be any object feen by a pcr- 

 Ibn in the medium ABCD. Then make QJE : ^E 

 and R F : r F : : fine of refraction to the fine of inci- 

 dence, a,nd the object will appear to be at q r nearly, 

 if the perfon was in the direction QJi produced. Let 

 O be the place of the perfon's eye in any other fitua- 

 tion, and join O r, O q, then the object is feen by rays 

 refracted within the furface mn, and Q^O, R;/O, arc 

 the directions nearly of the extreme rays by which the 

 object is vifible. 



As we are accuflomed to fee objects frequently 

 through thin panes of glafs, it may, to prevent milap- 

 prehenfions on this fubject, be neceffary to fhew what 

 changes take place in the apparent fituation of thefe 

 objects from the intervention of fuch a medium. 



Let ABCD (F?g. 15.) be a pane of glafs, QJ}. an 



object feen through it a whofe apparent place, fo.und by 



Q.3 the 



