Microscopical Essays, 



27 



t, A plane glass, one that is flat on each fide, and of an 

 equal thicknefs throughout. F, Fig. 13, Plate I. 



2. A double convex glass, one that is more elevated to- 

 wards the middle than the edge. B, Fig. 13, Plate I. 



3. A double concave is hollow on both fides, or thinner 

 in the middle than at the edges. D, Fig. 13, Plate I. 



4. A plano convex, flat on one fide, and convex on the 

 other. A, Fig. 13, Plate L 



5. A plano concave, flat on one fide, and concave on the 

 other. C, Fig. 13, Plate I. 



6. A meniscus, convex on one fide, concave on the other. 

 E,Fig. 13, Hate L 



It has been already obferved, that light proceeds invariably 

 from a luminous body, in ftrait lines, without the leaft devi- 

 ation ; but if it happen to pafs from one medium to another, it 

 always leaves the direction it had before, and affumes a new one. 

 After having taken this new direction, it proceeds in a ftrait 

 line, till it meets with a different medium, which again turns 

 it out of it's courfe. 



A ray of light, palling obliquely through a plane glafs, will go 

 out in the fame direction it entered, though not precifely in the 

 fame line. The ray C D, Fig. 4, Plate I, falling obliquely 

 upon the furface of the plane glafs AB, will be refracted towards 



the 



