30 



CONSTRUCTION OF THE MICROSCOPE. 



the eye, near which is a small arrow, to represent the 

 object under examination ; and the cones drawn from it 



Fig. 19. 



are portions of the rays of light diverging from those 

 points and falling upon the lens. These rays, if permitted 

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

 allow of their being brought to a focus upon the retina by 

 the dioptric media 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 

 precisely as if they had emanated directly from a larger 

 arrow placed at ten inches from the eye. The difference 

 between the real and the imaginary arrow is called the 

 magnifying power of the lens. 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. l 

 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, 



The Microscope. A microscope, as we have before ex- 

 plained, may be either a single, or simple^ or a compound 



(1) " The Mapnifyjng Power of Short Spaces " has been most ably elucidated by 

 John Gorham, Esq. M.R.C.S. See Journal of Microscopical Society, October. 



