and the Achromatism of Microscopes. 231 
to his edition of Ferguson’s Lectures. I ought not to omit that 
Robison mentions a translation in English of a work by Scherfer, 
which I have not been so fortunate as to meet with. 
When it is known that much attention has been bestowed 
on this theory by Clairaut, Euler, D’Alembert, and Boscovich, 
it may perhaps appear presumptuous to attempt any improve- 
ment on what they have done. I will therefore briefly describe 
the manner in which each of these mathematicians has con- 
sidered it, and thus, besides pointing out the defects of their 
operations, I shall have the advantage of more completely 
elucidating the subject. 
To explain the principle of the achromatic eye-piece, I will 
take its simplest case. Suppose a telescope composed of an 
object-glass and a single eye-glass, placed in the manner usually 
described under the head of Astronomical Telescope in most of 
our elementary works. The axis of a pencil of rays coming 
from the extreme visible point of an object, will pass through 
the center of the object-glass, and impinge on the eye-glass near 
its circumference, whence it will be refracted to the eye. But 
the dispersion attendant on this refraction will separate the ray 
into its differently coloured primitive rays. The violet pencil 
will be more refracted than the red, and will enter the eye, 
making with the axis of the telescope a greater angle than is 
made by the red rays. The place of the image therefore, as 
seen by the violet rays, is more distant from the center of the 
field of view than its place as seen by the red rays; and the 
object therefore appears coloured. But if a second eye-glass be 
placed at some distance from the first, the violet rays, after re- 
fraction at the first eye-glass, will be incident on the second at 
a point nearer to the center than that at which the red rays 
are incident on it; and falling therefore on a smaller refracting 
angle, they will, by the proper adjustment of the lenses, be so 
