460 RECAPITULATION, 



13. Microscopes are instruments for viewing 

 very small objects. They apparently magnify 

 objects, because they enable us to see them nearer, 

 without destroying distinctness of vision. 



14. Telescopes are instruments for viewing ob- 

 jects at a great distance. 



15. Acromatic telescopes are such as have 

 glasses so contrived, as to correct the unequal re- 

 fraction of the rays of light. 



Before we conclude the subject of optics, we 

 shall mention an improvement in spectacle-glasses, 

 by Dr. Wollaston. 



Supposing an eye to be placed in the centre of 

 any hollow globe of glass, it is plain that the ob- 

 jects would then be seen perpendicularly through 

 its surface in every direction ; consequently, the 

 more nearly any spectacle-glass can be made to 

 surround the eye, in the manner of a globular sur- 

 face, the more nearly will every part of it be at 

 right angles to the line of sight, the more uniform 

 will be the power of its different parts, and the more 

 completely will the indistinctness of lateral objects 

 be avoided. 



According to this principle, it appears that all 

 spectacle-glasses should be convex on their exte- 

 rior surface, and concave within. The section of 

 those for long-sighted persons will assume the 

 form of a meniscus, or crescent (Plate 17. %• 3.); 

 and those adapted for short sight, will have their 

 principal curvature on the concave side (Plate 17. 

 %. 4.) These glasses are called periscopic spec- 

 tacles, from their affording the opportunity of look- 

 ing round. 



