Chap. 7.] How 'very minute Oljeffs are made vijible. 247 



is an angle q L,p of a determinate magnitude*, which 

 varies indeed in, the eyes of different perfons, but is 

 fuch that any object making a lefs angle in the pupil 

 does not produce a lenfation on the retina of fuch a 

 nature that a perfon can have a determinate idea of 

 the object. This angle is in general about half a mi- 

 nute of a degree ; and each perfon may difcover the 

 ftrength of his fight by taking an object of a determi- 

 nate length, and removing it to fuch a diftance that 

 it mail juft ceafe to be vifible, or appear only as a 

 point. - 



When objects are beyond that diftance, they confe- 

 quently are invifible to us ; and if by any art we can 

 make them appear as if they were within this diftance, 

 they will then appear to us as other objects feen under 

 the fame angle, and at the fame diftance. The fimpleft 

 contrivance, where the object is either too minute or 

 too diftant, is to interpofe between the eye and the ob- 

 ject a glafs or lens, which by its magnifying power, 

 or, in ftill clearer terms, its power of converging the 

 rays of light, mall enlarge, or render more cbtule the 

 angle under which it is prefented to the naked eye, 

 and which muft of courfe proportionably enlarge the 

 image depicted on the retina. To find by what fort 

 of glafs or lens an object is to be feen at any diitance, 



* If the diflance of any object from the eye is fufficiently great 

 for the rays to fall nearly parallel on the pupil, the name objed is 

 feen more enlarged and dilHndl the nearer it is brought to the eye, 

 becaufethe image of any object on the retiaa will be gre.v 

 lefs in proportion to its apparent magnitud. : when the object is 

 top near the eye it continues to be enlarged, but is confufed. The 

 leaf: diftance is about fix inches. 



The eye is capable of difiinguifhing objedls that fubtend an 

 angle of half a minute of a degree, in which cafe the image on the 

 retina is lefs in breadth than the ^- r6 p.ut of an i = 

 object, fuppofing it fix inches difta'it 3 i-bout the Ti Vj part, of an 

 inch broad. And all fraaller objects are invifible to the naked eye. 

 R 4 we 



