on the Mechanism of the Eye. 31 
Corollary 11. Hence the line joining the remoter conju- 
gate foci, will always pass through the centre. The distance 
of the remoter focus of parallel rays will be expressed by 
f— — — — - ; and the least circle of aberration will be at the 
distance 7 — LlJLZLi Hi — _ dividing the length of aberration in 
the ratio of the distance of its limits from the surface. In the 
case of Cor. 10. /= — — — 
J z {mu — nty 
Corollary 12. This proposition extends also to reflected rays ; 
and, in that case, the line from the centre passes through the 
point of incidence. 
Proposition V. Problem , 
To find the place and magnitude of the image of a small 
object, after refraction at any number of spherical surfaces. 
Construction. (Plate II. Fig. 3.) From any point (B) in the 
object (AB), draw lines to (C), the centre of the first surface, 
and to (D), the focus of parallel rays coming in a contrary 
direction: from the intersection of the second line (BD) with 
the tangent (EF) at the vertex, draw a line (EH) parallel to 
the axis, and it will cut the first line (BC) in (H), the first 
image of the point (B). Proceed with this image as a new object, 
and repeat the operation for each surface, and the last point 
will be in the image required. For calculation, find the place 
of the image by Cor. 5. Prop. IV. and its magnitude will be to 
that of the object, as their respective distances from the centre. 
Corollary. If a confused image be received on any given 
plane, its magnitude will be determined by the line drawn from 
the preceding image through the centre of the last surface. 
