OPTICAL IMAGES. 
26. A plano-convex lens, fig. 15, has one side, A c, fiat, and 
the other convex. 
Fi?. 15. 
An 
A 
A 
Til 
n 
W 
1 
m 
c 
other, should be equal. The <3 
will depend on 
the radius o b 
A plano-concave lens, fig. 
16, has one side, A' c', fiat, 
and the other concave. 
A double convex lens, fig. 
17, has both sides convex, 
and a double concave lens, fig. 
18, both sides concave.. 
It is not necessary that the 
convexities of the sides in the 
one, or the concavities in the 
[•ee of convexity or concavity 
o’ w of the sphere of which 
F isr 1C. 
the lenticular surface is a part. The less that radius is, the 
greater will be the curvature of the surface. Thus, if o b be greater 
tliap. o' b', the sur- 
Flg,li ' face a' c' will be 
more convex (fig. 
17), or more con¬ 
cave (fig. 18), than 
__AC. 
°' A concavo-convex 
lens has one side, 
A c, fig. 19, concave 
and the other con¬ 
vex, the concavity, however, being greater than the convexity. 
A meniscus has also one side, A c, fig. 20, concave, and the 
other convex, but, on the contra^, the convexity is greater than 
the concavity. 
27. A line, o o', which joins the centres of the two lenticular 
surfaces in figs. 17, 18, 19, and 20, and which passes through the 
centre of the lenses, and one which, in figs. 15 and 16, is drawn 
from the centre o at right angles to the fiat surface, and passing 
through the centre of the lens, is callecl the axis of the lens. 
94 
