MAGNIFYING POWER OF LENSES. 17 



mountain, or even the whole visible surface of the 

 earth ; and in this case the apparent magnitude of 

 the pin's head is said to be equal to the apparent 

 magnitude of the mountain, etc. 



Let us now suppose the man c b, to be placed 

 at the distance of 100 feet from the eye at E, and 

 that we place a convex glass of 25 feet focal distance 

 half way between the object c b and the eye, that is, 

 50 feet from each ; then, as we have previously 

 shown, an inverted image of the man will be formed 

 50 feet behind the lens, and of the very same size 

 as the object, that is, six feet high. If this object 

 is looked at by the eye, placed six or eight inches 

 behind it, it will be seen exceedingly distinct, and 

 nearly as well as if the man had been brought 

 nearer from the distance of 100 feet to the distance 

 of six inches, at which we can examine mi- 

 nutely the details of his personal appearance. 

 Now, in this case, the man, though not actually 

 magnified, has been apparently magnified, because 

 his apparent magnitude is greatly increased, in the 

 proportion nearly of 6 inches to 100 feet, or of 

 200 to 1. 



But if, instead of a lens of 25 feet focal length, we 

 make use of a lens of a shorter focus, and place it in 

 such a position between the eye and the man, that 

 its conjugate foci may be at the distance of 20 and 

 80 feet from the lens that is, that the man is 20 feet 

 before the lens, and his image 80 feet behind it 

 then the size of the image is four times that of the 



