OPTICAL IMAGES. 



20. Thus, for example, if the transparent medium be water, the 

 depth of the image will be about three-fourths of the depth of the 

 object, and consequently water, when the bottom can be seen, 

 always appears less deep than it is in the proportion of 3 to 4. A 

 reservoir, whose real depth is 12 feet, will appear to have a depth 

 of only 9 feet. 



If the transparent body be glass, which has a greater refracting 

 power than water, in the proportion of about 8 to 9, an object 

 attached to the under- surf ace will appear to be at the depth of 

 about two-thirds of the thickness of the glass. 



21. If a rod L B I/, fig. 12, be plunged obliquely in water, it 

 will appear as if it were broken at B, the part immersed being 



-p io . 19 seen, not as it really is in the 



direction B L, but in the direc- 

 tion B /'. This will be easily 

 understood, when it is con- 

 sidered that the image of such 

 point of the rod will appear at 

 a less depth than the point 

 itself, in the proportion of 3 

 to 4. Thus the image of the 

 several points P will be at 

 the points ^>, the depths M p 

 being severally three-fourths of the depths M p. 



22. A certain part of the light which strikes upon the surface 

 of a transparent body will enter it, no matter what be the obliquity 

 with which it encounters it ; but there is a certain obliquity beyond 

 which light cannot emerge from it. Thus a ray of light proceed- 

 ing from any object under water, which strikes the surface at an 

 angle less than 41 32', cannot emerge, and in that case it may be 

 asked, what becomes of the ray ? The answer is, that it will be 

 reflected back into the water exactly as if the surface were a per- 

 fectly polished plane surface. 



In the same manner, if the transparent body be glass, the ray 

 cannot emerge from it, if the obliquity be less than 48 IT, and in 

 this case the ray will be reflected. 



The reflection which takes place under such circumstances, is 

 much more complete than any reflection from the surfaces of 

 bodies, whether naturally smooth or artificially polished. It has, 

 consequently, though somewhat improperly, been called PERFECT 

 REFLECTION, for, although the reflection is incomparably more 

 perfect than that from smooth or polished surfaces, nevertheless 

 there is still a small part of the light lost. 



The angle which limits the obliquity at which light can emerge 

 from a transparent body, is called the limit of transmission. 

 92 



