Epson Perfection V500 - Manual (Page 38)

a. transparency unit window
To place film for scanning, see Placing Film in the Holder .
To place slides for scanning, see Placing Slides in the Holder .
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Placing Slides in the Holder
You can scan up to four 35mm slides at a time using the slide portion of the film holder.
Note:
Make sure the white stickers on the back of the film holder and the areas
around them are not scratched, dusty, or covered in any way. If the areas
are obscured, the scanner may have trouble recognizing your slides.
Do not cover any of the small holes in the film holder or the slots on the
sides of the film holder.