APPLICATIONS OF OPTICAL KNOWLEDGE. 



71 



inches from the glass CD, and the eye applied at 

 the point S, the object AB will be seen turned 

 upside down, and magnified in the proportion of 



I to 12, or twelve times in length and breadth. 

 This forms what is called an Astronomical tele 

 scope ; but, as ever^ thing seen through it ap 

 pears inverted, it is not adapted for viewing ter 

 restrial objects. In order to fit it for viewing 

 land objects, two other eye-glasses, of the same 

 focal distance, (namely, one inch,) are requi 

 site ; the second eye-glass IK is placed at 2 

 inches from GH, or double their focal distance, 

 and the glass NO at the same distance from IK.* 

 By this means a second image IM is formed in 

 an upright position, which is viewed by the eye 

 at P. through the glass NO, and the object ap 

 pears magnified in the same proportion as before. 

 The magnifying power of a telescope of this 

 construction is found by dividing the focal dis 

 tance of the object-glass by the focal distance 

 of the eye-glass. Thus, if the object-glass be 

 36 inches in focal distance, and the eye-glass 



I 1 inch, the magnifying power will be 24 times ; 

 if the focus of the eye-glass be 2 inches, the 



Fig. 2. 



G 



H 



magnifying power will be 18 times, &c. LM 

 is the telescope fitted up for use. 



A compound microscope might likewise be 

 easily constructed by any ingenious artizan or 

 mechanic, by attending to the following illustra 

 tions and directions. Fig. 2 represents tho 

 glasses of a compound microscope. AB is thtr 

 glass next the object; CD is the amplifying 

 glass for enlarging the field of view; EF is the. 

 glass next the eye. When a small object, as 

 GH, is placed below the object-glass AB, at a 

 little more than its focal distance from it, a mag 

 nified image of this object is formed by the glass 

 AB at GH, which is magnified in proportion as 

 the distance .GG exceeds the distance of AG. 

 This magnified image of the object is magnified 

 a second time by the glass EF, to which the eye 

 is applied at K. This instrument, when fitted 

 up for use, is represented in fig. 3, where LM 

 represents a box or pedestal on which it stands, 

 Fig. 3. 



W 



This Is not the best form of a terr^trial eye 

 piece; but It may serve for the purpose of illustra- 

 tion. The eye-piece now most generally nsed. con 

 sists of four lenses, combined on a different prin 

 ciple. 



NO the stage on which the objects are placed, 

 over the opening i^which is supported by 3 pillars 

 fixed to the top of the box. P is a tube which is 

 supported by 3 pillars fixed into the stage. Into 

 this tube the tube R slides up and down for ad 

 justing the focus. The small tube u, which carries 

 the object-glass, is connected with the tube R, and 

 slides up and down along with it. S is the tube 

 which contains the two eye-glasses, and which 

 may be made to slide up and down into the tube 

 R, for increasing the magnifying power when oc 

 casion requires. T is a mirror, fixed on the pedes 

 tal, capable of moving up and down, and to th* 



