CONSTRUCTION OP THE MICROSCOPE. 23 



at once upon the pupil, would be too divergent to allow of 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 distinct vision. Thus altered, the eye receives them pre- 

 cisely as if they had emanated directly from a larger arrow placed at 

 ten inches from the eye. The difference between the real and the ima- 

 ginary arrow is called the magnifying power of the lens. Fig 19 will 

 perhaps convey a clearer idea of the increase of the angle of vision by 

 interposing a convex lens : 



without this lens placed at ^rf- 



/' g', the eye would see the ,..-■ 



dart at V c' under the ,.-■-''' 



angle formed by the eye A.--'"" 



and V c' ; but the rays h'f ^-^^'^^*' -""*" 



and eg from the extremi- ^^^^-<^^-' — ^^B 

 ties of the dart, in passing ^''^i::;-- _ ^B 



through the lens, are re- ^^'^^ ^W V ^ 



fracted to the eye in the ^ '~- 



direction of /"and g, which ""--..._ 



causes the dart to be seen ~~"--^, 



under the much larger fig. 19. 



angle formed with the eye 

 and de ; and it therefore appears so much magnified as to extend from 

 d to e. The object, when thus seen, appears to be magnified nearly in 

 the proportion which the focal distance of the lens bears to the distance 

 of the object when viewed by the unassisted eye ; and is entirely owing 

 to the object being distinctly viewed so much nearer to the eye than it 

 could be without the lens.* With these preliminary remarks as to the 

 medium by which microscopic power is obtained, we shall proceed to 

 apply them to the construction of a perfect instrument. A microscope, 

 as we have before explained, may be either a single or simple, or a com- 

 pound instrument. The simple microscope may consist of one, as seen 

 in fig. 18, or of two or three lenses ; but these latter are so arranged 

 as to have the effect only of a single lens. In the compound micro- 

 scope, not less than two lenses must be employed : one to form the in- 

 verted image of the object, which being the nearest to the object, is 

 called the object-glass; and the other to magnify this image, and from 

 being next the eye of the observer, called the eye-glass. Both these 



* " The Magnifying Power of Sliort Spaces" has received an able elucidation from 

 John Grorham, Esq. M.E.C.S. Journal 0/ Microscopical Society, October 1854. 



