292 OPTICS. 



and the other reclining it to either side, there re- 

 sults a twofold motion, which may easily be so 

 managed as to bring the glass to a right position ; 

 that is, to make it reflect the sun's rays directly 

 through the lens, upon the paper screen, and form 

 thereon a spot of light exactly round. 



Though the obtaining a perfect circular spot of light 

 upon the screen before you apply the microscope, 

 is a certain proof that the looking-glass is adjusted 

 right, yet that proof must not always be expected; 

 for the sun is so low in winter, that if it shine in a 

 direct line against the window, it cannot then 

 afford a spot of light exactly round. But if it be 

 on either side of you, a round spot may be ob- 

 tained, even in December. 



As soon as this appears, screw the tube C into 

 the brass collar provided for it in the middle of the 

 wood-work, taking care not to alter the looking- 

 glass; then screwing the magnifier you choose to 

 employ to the end of the microscope, in the usual 

 manner, take away the lens at the other end 

 of it, and place a slider, containing the object 

 to be examined, between the thin brass plates, as 

 in the other ways of using the microscope. 



Things being thus prepared, screw the body of. 

 the microscope to the short brass tube E, and pull 

 out the tube D, less or more, as the object is ca- 

 pable of enduring the sun's heat. Dead objects 

 may be brought within about an inch of the focus 

 of the convex lens 5 ; but the distance must be 

 shortened for living creatures, or they will soon be 

 killed. 



If the light falls not exactly right, you may 

 easily, by a gentle motion of the jointed wire and 

 pulley, direct it through the axis of the microscopic 

 lens. 



