66 THE TELESCOPE. 1 



Pleasure 's the portion of th' inferior kind, 

 But glory, virtue, Heaven for man designed. ; 



Barbauld. 



Questions. — 1. What has the microscope done for us ? 2. What '^ 

 is the appearance of grains of sand when examined by the eye, and  

 by the microscope' 3. Mouldiness ? 4. What is said of the green j 

 surface of standing water ? 5. What is the appearance of animalcules ] 

 in the infusions of pepper ? — now hay ? G. What appearance has the : 

 edge of a lancet ? 7. Sting of a bee ? 8. Fine lawn ? 9. Silk worm's j 

 web ? 1 



LESSON 39. 3 



I 

 The Telescope and Telegraph. 4 



Satellite, a small planet revolving round a larger, a moon. i 



Octag'onal, having eight angles and sides. i 



O'ral, delivered verbally, not written. ; 



No invention in the mechanic arts has ever proved more i 

 useful and entertaining than the production of the telescope ;-] 

 its utility both by sea and land is too well known to need 3 

 observation ; and without such assistance the science of as- 1 

 tronomy must have been far short of its present state. A 

 telescope is useful, not only for discovering tliose distantj 

 objects that are invisible to the naked eye, but for rendering j 

 more clear and distinct those that are discernible ; it is con- 3 

 structed to act either by refraction or reflection. It is the j 

 sole business of all telescopes to. enable the eye to see thes^ 

 object under a larger angle. For this purpose a new image of^ 

 an object is produced by the object-glass of the telescope, and i 

 then this image is viewed by means of the eye-glasses. Th©^;; 

 first impression, conveyed to the mind by a telescope, is that * 

 of bringing the object nearer, which is only another mode -J 

 of declaring that it is enlarged, or seen under a larger angle, j 

 To show objects in their natural posture, a telescope must ; 

 have three eye-glasses. The two additional lenses simply ] 

 give an erect position to objects. If you remove one of the J 

 eye-glasses from a common telescope, every thing wdl appear ' 

 in an inverted position. The three eye-glasses have all theif j 

 focal distances equal, and the magnifying power is found by | 

 dividing the focal distance of the object-glass by the focal ; 

 distance of one of the eye-glasses. The two additional lenses j 



