In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: A method permitting measurement of finger tremor as a displacement - time curve is described , using a test system with simple amplitude calibration .
### Output Text:
A method permitting measurement of finger tremor as a displacement - time curve is described , using a test system with simple amplitude calibration . 