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: Visual toxicity was of retinal origin and was characterized by a tritan - type dyschromatopsy , sometimes associated with a loss of visual acuity and pigmentary retinal deposits .
### Output Text:
Visual toxicity was of retinal origin and was characterized by a tritan - type dyschromatopsy , sometimes associated with a loss of visual acuity and pigmentary retinal deposits . 