rnr, MICROSCOPE. 7 



or the angle under which the object is seen, and apparent 

 brightness, or the angle of the pencil of light by which each of 

 its points is seen, and with these explanations we shall continue 

 to employ the common expressions magnifying glass and mag- 

 nifying power. 



The magnifying power of a single lens depends upon its focal 

 length, the object being in fact placed nearly in its principal 

 focus, or so that the light which diverges from each point may, 

 after refraction by the lens, proceed in parallel lines to the eye, 

 or as nearly so as is requisite for distinct vision. In Fig. 3, 

 A B is a double convex lens, near which is a small arrow to 



D. 



Fig. 3. 



represent the object under examination, and the cones drawn 

 from its extremities are portions of the rays of light diverging 

 from those points and falling upon the lens. These rays, if 

 suffered to fall at once upon the pupil, would be too divergent 

 to permit their being brought to a focus upon the retina by 

 the optical arrangements of the eye. But being first passed 

 through the lens, they are bent into nearly parallel lines, or 

 into lines diverging from some points within the limits of dis- 

 tinct vision, as from C and D. Thus altered, the eye receives 

 them precisely as if they emanated from a larger arrow placed 

 at D, which we may suppose to be ten inches from the eye, 

 and then the difference between the real and the imaginary 

 arrow is called the magnifying power of the lens in question. 



From what has been said it will be evident that two persons 

 whose eyes differed as to the distance at which they obtained 



