MAGNIFYING PO.WEK. 



retina at 1 1', but behind it at * i'. According to what has been 

 explained of optical images, the interposition of a lens, L L, of 

 suitable convexity, will bring forward the image from i i' to I i 7 , 

 and will therefore render the perception of the object distinct. 



Now, it is most important to observe in this case, that the 

 visual magnitude of the object, measured by the angle formed by 

 the lines o i and o' i', will be exactly the same as it would be if 

 the eye could have seen the object o o' without the interposition of 

 the lens : from which it appears that the lens does not, as is com- 

 monly supposed, directly augment the visual magnitude of the 

 object, but only enables the eye to see the object with distinctness 

 at a less distance than it could so see it without the interposition 

 of the lens. We say directly, because, although the lens does not 

 augment the visual angle of the object in the position in which it 

 is actually viewed, yet, by enabling the eye to see it distinctly at 

 a diminished distance, the visual angle of distinct vision, and 

 therefore the apparent magnitude of the object, is increased in 

 exactly the same proportion -as the distance at which it is viewed 

 is diminished. 



To understand the magnifying effect of the lens, we must con- 

 sider that the observer, seeing the object o o' with perfect distinct- 

 ness, obtains exactly the same visual perception of it as if the 

 object having the same visual magnitude were placed at that dis- 

 tance from the eye at which his vision would be most distinct. 

 Let the lines passing through the extremities of the object there- 

 fore be prolonged to this distance of most distinct vision, and let 

 an object, o o', be supposed to be placed there, similar in all re- 

 spects to the object o o', and having the same visual magnitude. 

 It will be evident, from what has been stated, that o o', as seen 

 with the lens, will have precisely the same appearance as the 

 object o o' would have if seen with the naked eye. The observer, 

 therefore, considers, and rightly considers, that the magnifying 

 power of the lens is expressed by the number of times that o o' is 

 greater than o o'; or, what is the same,by the number of times that 

 the distance of o 0' from the lens, that is the distance of most dis- 

 tinct vision, is greater than the distance of the object from the lens. 



It follows, therefore, generally, that the magnifying power of 

 the lens will be found by dividing the distance of most distinct 

 vision by the distance of the object from the lens. 



10. Adopting this method of estimating the magnifying power, 

 it would follow that the same lens would have different magni- 

 fying powers for different eyes, inasmuch as the distance of 

 most distinct vision for short sight is less than that for average 

 sight, and less for average sight than for far sight. 



To make this more clear, let E, fig. 2, represent an average 



101 



