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See Prism, Ð Achromatic telescope, or microscope, one in which the chromatic aberration is corrected, usually by means of a compound or achromatic object glass, and which gives images free from extraneous color.

This is conveniently done by cutting a large hole, the size of the brass cell of the object glass, through a sheet of corrugated strawboard, and slipping this on over the cell.

The object glass only of the telescope was used.

They make a cross line on my object glass.

"Except that I have replaced the object glass with the lens I picked up the other day at Thaneford's crow's-nest on Sugar Loaf."