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: Following already published methodology ( Eye 2002 ; 16 ; 567 - 571 ) monocular mean radial degrees ( MRDs ) to the I / 4e isopter on Goldmann perimetry was calculated for the right eye at the time of discovery of a visual field defect and again after not less than 18 months follow - up .
### Output Text:
Following already published methodology ( Eye 2002 ; 16 ; 567 - 571 ) monocular mean radial degrees ( MRDs ) to the I / 4e isopter on Goldmann perimetry was calculated for the right eye at the time of discovery of a visual field defect and again after not less than 18 months follow - up . 