4 THE MICKOSCOPE. 



its enabling the eye to approach more nearly to its object than 

 would otherwise be compatible with distinct vision. The com- 

 mon occurrence of walking across the street to read a bill is in 

 fact magnifying the bill by approach; and the observer, at 

 every step he takes, makes a change in the optical arrangement 

 of his eye, to adapt it to the lessening distance between him- 

 self and the object of his inquiry. This power of spontaneous 

 adjustment is so unconsciously exerted, that unless the atten- 

 tion be called to it by circumstances, we are totally unaware of 

 its exercise. 



In the case just mentioned the bill would be read with eyes 

 in a very different state of adjustment from that in which it 

 was discovered on the opposite side of the street, but no con- 

 viction of this fact would be impressed upon the mind. If, 

 however, the supposed individual should perceive on some part 

 of the paper a small speck, which he suspects to be a minute 

 insect, and if he should attempt a very close approach of his 

 eye for the purpose of verifying his suspicion, he would pre- 

 sently find that the power of natural adjustment has a limit; 

 for when his eye has arrived within about ten inches, he will 

 discover that a further approach produces only confusion. But 

 if, as he continues to approach, he were to place before his eye 

 a series of properly arranged convex lenses, he would see the 

 object gradually and distinctly increase in apparent size by the 

 mere continuance of the operation of approaching. Yet the 

 glasses applied to the eye during the approach from ten inches 

 to one inch, would have done nothing more t&an had been pre- 

 viously done by the eye itself during the approach from fifty 

 feet to one foot. In both cases the magnifying is effected 

 really by the approach, the lenses merely rendering the latter 

 periods of the approach compatible with distinct vision. 



A very striking proof of this statement may be obtained by 

 the following simple and instructive experiment. Take any 

 minute object, a very small insect for instance, held on a pin or 

 gummed to a slip of glass; then present it to a strong light, 

 and look at it through the finest needle-hole in a blackened 

 card placed about an inch before it. The insect will appear 

 quite distinct, and about ten times larger than its usual size. 

 Then suddenly withdraw the card without disturbing the ob- 



