...and roore recently achromatic illuminators have been constructed by most of our instrument makers. Some years since, Mr. Gillett was led by observation to appreciate the importance of controlling uot merely the quantity of light which may be effected by a diaphragm placed anywhere between the source...

...employed, and more recently achromatic illuminators have been constructed by most of our instrumentmakers. Some years since Mr. Gillett was led by observation...placed immediately behind the achromatic illuminating combination. An elastic diaphragm, or artificial pupil, as it might be called, was first proposed by...