A reading glass is a low power magnifier that uses a
biconvex lens: this is a clear disc made of either plastic
or glass with a convex surface on both sides.
If you take a biconvex lens and place it close to an
object so that it lies within its front focal point, you
generate an image of the subject from light reflected
from its surface. This is seen when looking through the
opposite side of the lens. When viewed in this
way, the subject appears enlarged and right side up.
Even at this low power, a critical observer can see optical
artefacts. One such is chromatic (colour) aberration,
which is when a border displays colour fringes, a
defect caused by the lens acting like a prism. White
light is a composite of all the colours of the rainbow,
where each colour corresponds to a specific wavelength
of light. When white light passes through a
lens or a prism, it separates to its component colours
as the different wavelengths are bent to differing
degrees. Thus, a lens will bring the red light rays to a
more distant focus point than the blue rays, and this
creates a colour fringe at the borders of the subject.
This defect can be corrected by using a lens that
consists of two types of glass with different optical
properties. Such a lens is described as being ‘achromatic’.
In the case of the lower-powered reading glass
the colour fringes resulting from chromatic aberration
are not sufficiently large to disturb most viewers.
However, at higher magnification, the subject’s fine
borders will show these colour fringes and will detract
from the appearance of the image.
An example of an achromatic loupe is the Hastings
Triplet Magnifier. Superficially this looks like a single
lens, but it is really a composite lens made up of three
elements cemented together. In this manner, the
magnifier corrects for chromatic aberration and
improves the optical quality of the image by rendering
the specimen’s borders without the colour fringes.
High quality magnifiers, such as the Hastings Triplet,
are a worthwhile addition to any photomicrographer’s
arsenal. They will be used to evaluate specimens before
using higher power examination with the microscope.