2O4 Real and apparent rifmg of the Sun> &JV. [Book III. 



the bafon ; then by pouring water into the bafon, the 

 fhilling inftantly appears 5 for by what has been faid 

 above, the object, being now in a denier medium, is 

 fnade to appear nearer to its furface. 



As the refraction muft in all cafes depend on the ob- 

 liquity of the ray, that part of any body which is moft 

 immerfed will feem to be moft materially altered by 

 the refraction. When, however, the object extends to 

 no great depth in the water, the figure is not materially 

 diftorted; but if the object is of a considerable fizc, or 

 extends to a great depth, thofe rays which proceed 

 from the more diftant extremities come in a more 

 oblique direction on their emergence into the air, and 

 they confequently fuffer a greater refraction than the 

 reft. Thus a ftrait leaden pipe appears near the bottom 

 of a deep water to be curved, and a flat bafon feems 

 deeper in the middle than near the fides. 



To thefe laws of refraction is to be attributed the 

 difference between the real and the apparent rifmg of 

 the fun, moon, and ftars, above the horizon. The ho- 

 rizontal refraction is fomething more than half a de- 

 gree, whence the fun and moon appear above the ho- 

 rizon when they are entirely below it. From the ho- 

 rizon the refraction continually decreafes to the zenith. 

 Refraction is incrcafed by the denfity of the air, and 

 confequently it is greater in cold countries than in hotj 

 and it is alfo affected by the degree of cold or heat in 

 the fame country. 



Parallel rays, if refracted, preferve thtir parallel dU 

 rection both in entering and in pafiing out of a refract- 

 ing medium, provided the two fur faces of the refract- 

 ing medium are parallel. The two ray*, E A, EA, 

 (fig. 4.) after refraction, while they approach the per- 

 pendiculars pp, continue parallel as before, the reafon 

 of which is evident on the principles already efhblilh- 



