332 
will be caused to meet at a point, called the 
Jocus, some distance beyond the centre of cur- 
vature, This effect will not be materially 
changed, by allowing the rays to into air 
again through a plane surface of glass, such as 
would be formed by a section of the glass in the 
vertical line ; a lens of this description is called 
a plano-conver lens ; and it will hereafter be 
shown to possess properties, which render it 
very useful in the construction of microscopes. 
But if, instead of passing through a plane sur- 
face, the rays re-enter the air through a corivex 
surface, they will be made to converge still more. 
This may be best understood by considering 
the course of parallel rays, as in the adjoining 
Fig. 145. 
F ra: 
NTS _ Le 
l eal = 
ALK ee 
Bai ee eC 
in ee 
aS 
a: § 
A B, parallel rays passing through glass and falling 
upon the convex surface FBG; BH, B :: 
radii prolonged, which are the perpendiculars to 
the curved surface at the several points; BC, 
course of the rays if unrefracted; B E, their 
course in consequence of refraction. 
figure (fig. 145). Here the radii pone will 
be the perpendiculars to the curved surface; and, 
according to the law of refraction just alluded to, 
the rays passing from the dense into the rare 
medium will be bent from the perpendicular, 
so as to be made to converge towards a focus, 
as in the former instance. It is easy to see, 
therefore, that the effect of the second convex 
surface will be precisely equivalent to that of 
the first; for the contrary direction of the sur- 
face is antagonized by the contrary direction of 
the refraction; so that the focus of a double 
convex lens will be at just half the distance 
from it, or (as commonly expressed) be half 
the length of the focus of a plano-convex lens. 
In fact, the focus of the former to parallel rays 
will be the centre of its sphere of curvature, and 
its focal length will therefore be the radius; 
whilst the focus of the latter will be in the op- 
posite side of the sphere, and its focal length 
will be the diameter. Now it is evident that 
Fig. 146. 
a falling on a double convex lens brought to 
focus ve paced “eg pres. che ca 
MICROSCOPE. 
Fig. 147. 
Parallel falling on a plano-convex lens brought to 
a Seed al che distance of ita diandter, aa 
versa. . 
if a double convex lens will bring parallel rays 
to a focus in the centre of its sphere of curva- 
ture, it will on the other hand cause rays to 
assume a parallel direction, which are diverging 
from its focus; so that if a luminous body were 
placed in that point, all its cone of rays, which 
fell upon the surface of the lens, would pass 
out in a cylindrical form. Again, if rays al- 
ready converging fall upon a convex lens, they 
Fig. 148. 
Rays already converging brought to a focus nearer than 
the centre; and rays di ing from such a point, 
still diverging in a diminished . 
Fig. 149. 
Rays divergi i distant than the 
nh y ip peta ry 
yond it, ; 
nearer to 
will be brought to a focus at a point 
it than the focus for parallel rays (which is 
called its principal focus); and, if they be di- 
verging from a distant point, their focus will be 
more distant than the principal focus. The 
further be the point from which they diverge, 
the more nearly will the rays approach the pa- 
rallel direction; until, at length, when the ob- 
jects are very distant, their rays in effect become 
parallel, and are brought to a focus in the 
centre of the sphere. If they diverge from the 
other extremity of the diameter of the sphere, 
they will be brought to a focus at a nd. 
ing distance on the other side of the lens. On 
= >" 
the other hand, if they be diverging from a point | 
