SciENiiFic Lectures. 35 



througli media bounded bj parallel faces, continue to be parallel. 

 Diverging or converging rays, under similar circumstances, become 

 more diverging or less converging. If, before transmission, they 

 were diverging from a single radiant, they no longer have this 

 character after transmission, but are said to be affected by aherra- 

 tion. This effect sensibly disturbs the performance of the micro- 

 scope when used to observe objects covered, as many are, with thin 

 glass plates, and requires a special contrivance to counteract it. 

 Parallel rays falling on media having faces inclined to each other 

 will be unequally refracted by the different faces, and may be made 

 relatively to diverge or converge after transmission. It is evident that 

 the faces may be so multiplied and so adjusted to each otlier as to make 

 all the transmitted rays intersect in a single point. To make this rigor- 

 ously true, the faces must be infinite in number, or the surface must 

 be a curve. It is not difiicult to determine mathematically what 

 curve or curves are proj)er to produce this effect. An ellipse will 

 do it. Elliptic curves are not, however, used in the construction of 

 the microscope. 



The name lens is given to solid media employed in optics to concen- 

 trate or scatter rays of light. Only spherical curvatures are given to 

 lenses. Some disadvantages attend this curvature, but they are, in prac- 

 tice, effectually overcome ; and it possesses advantages, on the other 

 hand, which belong to no other. Tlie points of intersection of convergent 

 rays are called foci. The focus of parallel rays is the principal focus. 

 The term radiant is applied to a fomis from which rays diverge. 

 Kays fi-om a radiant more distant than the principal focus of a con- 

 vex lens will converge to a second focas on the other side of the lens. 

 The two are called conjugate foci. Kays from a point nearer to the 

 lens than its principal focus will emerge divergent, but less divergent 

 than before, as if they proceeded from a more distant radiant. 

 This more distant radiant is called the imaginary or negative 

 focus, and is still the conjugate to the original radiant. With 

 concave lenses, the effects are all the reverse of those just 

 described. If, instead of a single radiant, an object of sensible 

 magnitude be placed before a convex lens, more distant than 

 its principal focus, every point of this object may be considered an 

 independent radiant, having its corresponding conjugate focus, and 

 these conjugate foci will combine to produce an image of the object, 

 which will be inverted and reversed. If the object be nearer than 

 the principal focus, as all the emergent rays will be divergent, no 



