ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
115 
Now, in so far as tlie amplification of the image projected by the 
objective is concerned, the distance of distinct vision is of no consequence 
whatever, but the result is governed solely by the well-known optical 
law that, “ The linear dimensions of object and image are directly as 
their distances from the optical centre of the lens.” The correctness of 
this can be demonstrated by actual measurement, for the image is as real 
as the object, and its distance from the optical centre of the lens and its 
dimensions can as easily be measured. 
Before proceeding to the further discussion of this subject it may 
be well to define some of the optical terms which I shall be obliged to 
use. I am quite aware that, for the majority of my hearers, this is a 
work of supererogation that almost savours of impertinence ; but there 
are always beginners amongst us, and it is for their sakes that I insert 
these elementary definitions. 
Definitions. — Optical Centre. — The point through which all rays 
traversing a lens with parallel directions at incidence and emergence 
must pass. In double convex or double concave lenses it lies in the 
interior of the lens; in plano-convex or plano-concave lenses it lies on 
the curved surface ; while in a meniscus of either kind it lies outside 
the lens altogether. 
Principal Axis. — The straight line passing through the centres of 
curvature of both faces of a double convex, a double concave, or a 
meniscus lens, or passing through the centre of curvature of the curved 
face and cutting at right angles the plane face of a plano-convex or a 
plano-concave lens, is called the Principal Axis ; the optical centre is 
always in this line. 
Secondary Axes. — All other straight lines passing through the 
optical centre are called “ Secondary Axes.” 
Principal Focus. — The point at which rays originally parallel to the 
principal axis are made to converge (approximately) to one point. 
Focal Length. — The distance from the optical centre to the principal 
focus. 
Conjugate Foci. — Rays emerging from a point more distant than 
the principal focus on one side of a convex lens and passing through 
the lens will be brought to a focus at a point on the other side of the 
lens, and the points thus related are called conjugate foci. 
As one conjugate focus advances from infinite distance (parallel 
rays) to the principal focus, the other recedes from the principal focus 
to infinite distance, the most distant focus always moving most rapidly, 
and the least distance between them is therefore attained when they are 
equidistant from the optical centrf , in which case the distance of each 
from the optical centre is C Z /, and their distance from each other 4 /. 
If either is less than the principal focus, then the other becomes negative ; 
that is, the rays are no longer brought to a focus on the opposite side of 
the lens, but are only rendered less divergent, as if coming from a more 
distant point on the same side, and this point from which they appear 
to come (the more distant of the two) is called the virtual conjugate 
focus. In this case, as one conjugate focus advances towards the optical 
centre, the other advances in the same direction till they become 
coincident. 
Secondary principal and conjugate foci exist in each of the secondary 
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