HOW TO WORK WITH THE TELESCOPE. 
285 
The curvature of the image is the least important of the 
three defects named — a fortunate circumstance, since this 
defect admits neither of remedy nor modification. The image 
of a distant object instead of lying in a plane, that is, forming 
what is technically called a flat field, forms part of a spherical 
surface whose centre is at the centre of the object-glass. 
Hence the centre of the field of view is somewhat nearer to 
the eye than are the outer parts of the field. The amount of 
curvature clearly depends on the extent of the field of view, 
and therefore is not great in powerful telescopes. Thus if we 
suppose that the angular extent of the field is about 30' (a 
low-power, and therefore large field), the centre is nearer than 
the boundary of the field to the eye by about g-|-gth part only 
of the fields diameter. 
The indistinctness of the field is partly due to the obliquity 
of pencils forming the image, and partly to spherical aberration. 
The first cause cannot be modified by the optician's skill, and 
is not important when the field of view is small. Spherical 
aberration, owing to which those parts of a direct pencil which 
fall near the boundary of a convex lens, converge to a nearer 
focus than those which fall near the centre, may be corrected 
by a proper selection of the forms of the two lenses which 
replace in all modern telescopes the single lens hitherto 
considered. 
The false colouring of the image is due to chromatic aberra- 
tion. The pencil of light proceeding from a point, converges, 
not to one point, but to a line of varying colour. Thus a 
series of coloured images, at different distances 
from the object-glass, is formed ; sp that if a screen 
were placed to receive the mean image in focus, a 
coloured fringe, due to the other images {out of 
focus, and therefore too large), would surround the 
mean image. 
Newton supposed that it was impossible to get 
rid of this defect, but the discovery that the dis- 
persive power of a medium is not proportional to 
its refractive power, supplied opticians with the 
means of correcting the confusion of coloured 
images. This is effected by combining, as shown 
in fig. 4, a convex lens of crown glass with a con- 
cave lens of flint glass, the convex lens being nearest to the 
object. A little colour still remains, but -not enough seriously 
to affect the distinctness of images. 
But even if the image formed by the object-glass were 
perfect, yet this image, viewed through a single convex lens of 
short focus (placed as in fig*. 1), would appear curved, indis- 
tinct, coloured, and also distorted , because viewed by excen- 
