THE MICROSCOPE. 



distinct vision, would give different results as to the magnify- 

 ing power of a lens. To one who can see distinctly with the 

 naked eye at a distance of five inches, the magnifying power 

 would seem and would indeed be only half what we have 

 assumed. Such instances are, however, rare; the focal length 

 of the eye usually ranges from six to twelve or fourteen inches, 

 so that the distance we first assumed of ten inches is very near 

 the true average, and is a convenient number, inasmuch as a 

 cipher added to the denominator of the fraction which expresses 

 the focal length of a lens gives its magnifying power. Thus a 

 lens whose focal length is one-sixteenth of an inch is said to 

 magnify 160 times. 



When the focal length of a lens is very small, it is difficult to 

 measure accurately the distance between its centre and its 

 object. In such cases the best way to obtain the focal length 

 for parallel or nearly parallel rays is to view the image of some 

 distant object formed by the lens in question through another 

 lens of one inch solar focal length, keeping both eyes open and 

 comparing the image presented through the two lenses with 

 that of the naked eye. The proportion between the two 

 images so seen will be the focal length required. Thus if the 

 image seen by the naked eye is ten times as large as that shown 

 by the lenses, the focal length of the lens in question is one- 

 tenth of an inch. The panes of glass in a window, or courses 

 of bricks in a wall, are convenient objects for this purpose. 



In whichever way the focal length of the lens is ascertained, 

 the rules given for deducing its magnifying power are not 

 rigorously correct, though they are sufficiently so for all prac- 

 tical purposes, particularly as the whole rests on an assumption 

 in regard to the focal length of the eye, and as it does not in 

 any way affect the actual measurement of the object. To cal- 

 culate with great precision the magnifying power of a lens with 

 a given focal length of eye, it is necessary that the thickness of 

 the lens be taken into the account, and also the focal length of 

 the eye itself. 



We have hitherto considered a magnifying lens only in refer- 

 ence to its enlargement of the object, or the increase of the 

 angle under which the object is seen. A further and equally 

 important consideration is that of the number of rays or quan- 



