in optics, a sort of telescope, peculiarly fitted for viewing the sun without hurting the eyes. See Telescope.

As the sun may be viewed through coloured glasses without hurt to the eyes, if the object and eye glasses of a telescope be made of coloured glass, as red or green, such a telescope will become an helioscope.

But Mr Huygens only used a plain glass, blacked at the flame of a candle on one side, and placed between the eye-glass and the eye; which answers the design of an helioscope very well.