Achromatic: free from color; tissue that does not stain readily.
An achromatic grating, however, is (rashly speaking) an impossibility.
The bands are then achromatic in the sense that the ordinary telescope is so.
If not, the substitution of an achromatic lens will be of no advantage.
Given the achromatic object-glass, why should not it be divided?
The achromatic doublets ...