Heinrich Geissler (1814-1879) was a highly regarded worker in glass instruments. He made quality thermometers with precisions of 0.01 to 0.04°C.
This thermometer is unusual because its scale runs from -1 to +0.5° and then after a thickening in the capillary from 9 to 20°. The divisions are at 0.02° intervals.
The thermometer is dated 1897 and must be from his company continuing to operate after his death.
The etching of the eagle on the glass looks like some certification mark, but no information is available about this.

Inscriptions / Translations:  Internally on back of scale: Celsius Dr H. Geissler Nachf Frz. Müller in Bonn
Etched on glass on back: Nr 9431 (Symbol of eagle) 1897
Label on case: FRANZ MÜLLER Dr H. Geissler Nachf BONN a/Rhein