384 optics. 



to their foci, and the distribution of the light on 

 the object, are what require the most attention. 

 These microscopes are sometimes fitted up with a 

 rack and pinion, to the pillar A and pin D, for the 

 more ready adjustment of the glasses to their 

 proper foci. 



The following is a single microscope on a foot 

 or stand. AB (Fig. 11.) is the basis or foot, C I 

 is the stem ; EF, are two square sockets of brass,, 

 moveable up and down together upon the square 

 part of the stem, being connected by a common 

 screw ; but this motion is checked by the constant 

 pressure of a spring. G is a screw, by which the 

 part E is fixed to the stem. 



H is an adjusting screw, by which the part F is 

 gradually moved up and down ; and thereby K L, 

 the stage on which the objects are placed, has 

 M N, the slider, with its objects, duly adjusted to 

 the focus of the magnifiers. 



O is the circular piece of brass, on which are 

 fixed three lenses turning on centres fixed in O, 

 and which may be used either separately, or com- 

 bined together ; R is the speculum, that reflects 

 the light through the microscope. 



A megalascope is an optical instrument, adapted 

 for viewing all the larger sort of small objects \ 

 such as insects, flowers, minerals, linen, &c. to a 

 very great advantage ; as, with three glasses only, 

 it has seven different magnifying powers. 



A (Fig. 12.) is the case of brass, silver, &c. ; D, 

 E, F, three several lenses with different magnify- 

 ing powers; which are all contained in the case, 

 and turned out at pleasure : H the handle. 



The three glasses, singly, afford three magnify- 

 ing powers ; and, by combining two and two, we 

 make three more : for D with E makes one, D 



