THE TELESCOPE. 4G7 
are removed from view even when the distance of these remote 
objects is diminished by that amount. 
When the telescope (whether that discovered by the Tuscan 
artist or German optician) fust revealed the glories of the 
heavens, it was imagined that by the simple combination of 
an eye-lens and a single object-lens, the problem was com- 
pletely solved, and that the principle at least could not be 
improved. It was seen indeed that it gave indistinct and 
coloured images, and these latter for a long course of years 
baffled every endeavour to correct them, and it was not until 
a century and a half after the invention of the telescope that 
they were finally removed, and that objects seen through this 
instrument appeared perfectly colourless and well defined. The 
causes of the imperfection of the original telescope were that the 
rays of light could not be made to meet at the same focus, and 
this was also the case with the colours of which the original 
ray of white light is composed, and which was decomposed, as 
by a prism, into its original parts by its passage through the 
lens. These are technically called aberrations or deviations — 
the first, resulting from the figure of the object-lens, is termed 
the spherical ; the latter, which depends on the glass itself (all 
homogeneous transparent media being affected by this cause), 
is called the chromatic aberration. 
The spherical aberration arises from the circumstance that 
of the parallel rays passing through the lens AB (fig. 1), those 
near the margins of the glass will be bent or refracted to the 
focus/, whilst those passing nearer the centre of the lens will 
arrive at their focus at the point /'. The distance between those 
points is termed the longitudinal spherical aberration. If we 
look at the image of the sun thrown through a convex 
glass upon a dark 
screen, it will be 
noticed that in- 
stead of a point of 
light, the so-called 
focus will be sur- 
rounded by a halo. 
But if the outer 
margins of the 
lens be hidden by 
an annular diaphragm, it will be seen that less haziness exists 
and the focus becomes more distinct at the point/'. By giving 
certain curvatures to the lens the spherical aberration may 
be greatly diminished, although not altogether got rid of by 
using a single lens, at least by one having spherical surfaces. 
For instance, in a plano-convex lens with the plane side turned 
towards the parallel rays, the distance //' is four and a half 
VOL II. — HO. VIIT. 2 K 
A 
Fig. 1, 
