Trief, Ellen; Bruce, Cascella (August 2010). "The selection of tangible symbols by educators of students with visual impairments and additional disabilities". Journal of Visual Impairment & Blindness. 104 (8): 499–504. doi:10.1177/0145482X1010400806. S2CID142826867.
Trief, Ellen; Bruce, Cascella (August 2010). "The selection of tangible symbols by educators of students with visual impairments and additional disabilities". Journal of Visual Impairment & Blindness. 104 (8): 499–504. doi:10.1177/0145482X1010400806. S2CID142826867.