280 OPTICS. 



Fig. 9. is a tube of glass, contrived to confine 

 living objects, such as frogs, fishes, &c. in order to 

 discover the blood as it flows along the veins and 

 arteries. 



All these particulars are contained in a little 

 neat box, very convenient for carrying in the 

 pocket. 



When an object is to be viewed, thrust the ivory 

 slider, in which the object is placed, between the 

 two flat brass plates, E E ; observing always to put 

 that side of the slider where the brass rings are, 

 farthest from the eye. Then screw on the mag- 

 nifying glass you intend to use, at the end of the 

 instrument G: and looking through it against the 

 light, turn the long screw C, until the object be 

 brought to fit your eye ; which you will know by 

 its appearing then perfectly distinct and clear. It 

 is best to look at it first through a magnifier that 

 can show the whole of it at once, and afterwards 

 to inspect the several parts more particularly with 

 one of the greatest magnifiers j for thus you will 

 gain a true idea of the whole, and of all its parts. 

 And, though the greatest magnifiers can show but 

 a minute portion of any object at once, yet, by 

 gently moving the slider that contains the object, 

 the eye will gradually examine it all ; and if any 

 part should be out of distance, the screw C will 

 easily bring it to the true focus. 



As the object must be brought very near the 

 glasses when the greatest magnifiers are used, be 

 particularly careful not to scratch them as you 

 move it in or out. A few turns of the screw C 

 will easily prevent this mischief, by giving room 

 enough. 



You may change the objects' in your sliders for 

 what others you think proper, by taking out the 



