NEGATIVE EYE-PIECE NEARLY ACHROMATIC. 
17 
Sixthly : the red and blue rays, after being refracted by the 
eye-lens, arrange themselves in such relation to the lines a 0 
and a' 0'\ that they all appear to proceed from a single straight 
image, entirely free from both chromatic and spherical aber- 
ration. The rays B 0 and R 0 not actually coincide in 
position, but they appear to emanate from the same point, 
while other rays, emanating from the same point in the object, 
so over-lie these as to unite in every position all the colors of 
the spectrum, giving perfectly white light and achromatic vis- 
ion of the object. 
29. Advantage of over-correcting tlie Object-Olass. 
The eye-lens E E has its focus for red rays longer than its 
focus for blue rays. If the object-glass had not been over- 
corrected, the action of the field-lens would have caused the 
blue image to be formed at 5, and the red image at but the 
over-correction of the object-glass has placed the blue image 
as much nearer to the eye-lens than the red, as is required by 
the diiference between its foci for blue and red rays, and the 
curvature of the images which has been reversed by the field- 
lens, just equals the aberration of sphericity of the eye-lens, so 
that it appears to the observer free, also, from this species of 
error, as shown by the magnified straight image which the eye 
sees situated at 0 O". 
30. IVegative Eye-piece nearly Achromatic. Let us 
now examine the action of the lenses F F and E E, a little 
more particularly. The pencil of rays from the centre of the 
object is so condensed by the object-glass AMP, that it occu- 
pies but a small space about the central portion of the field- 
lens F F. The border rays of this pencil, after passing the 
field-lens, cross each other, the red rays in the red image K' E', 
and the blue rays in the blue image B', and after thus cross- 
ing, they impinge upon the eye-lens each ray on the opposite 
side of the axis from what it was when refracted by the field- 
lens ; thus the lateral chromatic aberration of the field-lens will 
be nearly corrected by the eye-lens, but as the eye-lens has a 
shorter focus than the field-lens, its chromatic aberration will 
be in excess. 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
2 
