Thus , the missing notes indicated the positions where black ink was on the page and could be used to read .
Each note corresponded with a position on the optophone 's reading area , and that note was silenced if black ink was sensed .
The missing notes indicated the positions where black ink was on the page and could be used to read .
Each note corresponded with a position on the optophone 's reading area , and that note was silenced if black ink was sensed .