san be selected, e.g., which may have the same refraction, but different 
dispersion, and vice versa. It is just this property that makes it 
possible to combine two or more lenses together to counteract this 
defect, which is characteristic of all simple lenses. We shall refer to 
this again, but let us discuss another inherent defect of the simple lens. 
This is known as spherical aberration, and is due to the use of spherical 
surfaces for lenses, as these are the only ones that can be ground 
accurately. It may be described as the inability of a lens to convey 
the rays passing through the margins to a focus at the same distance 
from the lens as the central rays. A ray passing direct through the 
centre is brought to a focus at a greater distance than a parallel ray 
passing through near the edge. Hence, if the sun is focussed on a 
screen by a single lens, there is no single bright point of light produced 
as an image, but a series of dises of light. This can always be reduced 
in a single lens by using a “stop” which cuts out the more marginal 
rays. One can readily prove this by cutting a circle of thick brown paper 
to fit just inside the rim of a reading glass, and then cutting a central 
hole in the paper, amounting to, say, half or less than the whole 
diameter, and observing some print through the glass, with and without 
the stop. By keeping the eye and the print fixed, and alternately using 
the centre of the glass, through the aperture in thé paper, and then 
covering the centre over with the piece cut out and looking through the 
edges only, this difference in focus will be readily seen by having to 
move the lens to bring the print into focus through the margin. 
Now, a bi-convex lens will focus light to a focal point, and so also 
will a lens known as a plano-convex, i.e., convex one side and flat on 
the other. Lenses are also made bi-concave, i.c., concave both sides, 
consequently thinnest at the centre and thickest through the edges. 
These lenses, instead of bringing parallel rays of light to a focus, will 
make the rays diverge. A similar result is obtained by a plano-concave 
lens. It is now possible to combine different kinds of glasses, and 
different shaped lenses, in such numerous ways as to counteract the 
chromatic and spherical aberrations to a very large extent. 
It was only about 150 years ago that anything except crown glass 
was available to the lens maker. Optical glass is only about that 
number of years old. A new type was introduced in flint glass, having 
a greater refraction and still greater dispersion than the old crown glass. 
It was made from light to heavy, and in the majority of lenses to-day 
these crown and flint glasses are to be found. 
It is now a common practice to make compound lenses of a convex 
lens of crown glass (+ lens) and a concave lens of flint glass (— lens), 
which are achromatic, on account of the large correction of the chromatic 
aberration. 
Two colours are made to come to the same focal point. These are 
usually, for ordinary visual work, a part of the red orange spectrum, 
and a part of the greenish-blue region. For photographic work it is 
the yellow and blue rays. 
As long ago as 1762, Euler began a discussion on the theory of the 
achromatic microscope. In 1811 Fraunhofer made achromatic 
doublets, and by 18283 there were microscopes with high powers of four 
doublets screwed together, and capable of magnifying up to 1,200 times. 
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