THE MICKOSCOPE. 5 



ject, which will instantly become indistinct and nearly invisi- 

 ble. The reason is, that the naked eye cannot see at so small a 

 distance as one inch. But the card with the hole having en- 

 abled the eye to approach within an inch, and to see distinctly 

 at that distance, is thus proved to be as decidedly a magnifying 

 instrument as any lens or combination of lenses. 



This description of magnifying power does not apply to such 

 instruments as the solar or gas microscope, by which we look 

 not at the object itself, but at its shadow or picture on the wall; 

 and the description will require some modification in treating 

 of the compound microscope, where, as in the telescope, an 

 image or picture is formed by one lens, that image or picture 

 being viewed as an original object by another lens. 



It is nevertheless so important to obtain a clear notion of the 

 real nature of the effect produced by a lens applied to the eye, 

 that we will adduce the instance of spectacles to render the 

 point more familiar. If the person who has been supposed to 

 cross the street for the purpose of reading a bill had been aged, 

 the limit to the power of adjustment would have been discov- 

 ered at a greater distance, and without so severe a test as the 

 supposed insect. The eyes of the very aged generally lose the 

 power of adjustment at a distance of thirty or forty inches 

 instead of ten, and the spectacles worn in consequence are as 

 much magnifying glasses to them as the lenses employed by 

 younger eyes to examine the most minute objects. Spectacles 

 are magnifying glasses to the aged because they enable such 

 persons to see as closely to their objects as the young, and 

 therefore to see the objects larger than they could themselves 

 otherwise see them, but not larger than they are seen by the 

 unassisted younger eye. 



In saying that an object appears larger at one time, or to one 

 person, than another, it is necessary to guard against miscon- 

 ception. By the apparent size of an object we mean the angle 

 it subtends at the eye, or the angle formed by two lines drawn 

 from the centre of the eye to the extremities of the object. 

 In Fig. 1, the lines A E and B E drawn from the arrow to the 

 eye form the angle A E B, which, when the angle is small, is 

 nearly twice as great as the angle C E D, formed by lines drawn 

 from a similar arrow at twice the distance. The arrow A B 



