A traditional slide viewer does not invert anything. It simply shines light through a transparency so you can see it.

That means:

• A color slide (positive transparency) appears as a normal positive image.
• A negative film frame will still look like a negative if you put it in a regular slide viewer.

If you’ve seen a device that shows negatives as positive-looking images, that’s a different kind of viewer. Those negative viewers typically use a camera/sensor and a display, then electronically invert the tones and colors so the image looks positive on screen.

So the key distinction is:

• Slide viewer = optical viewing only, no inversion
• Negative viewer/scanner = electronic viewing with inversion

Also, “slide” usually refers to a positive transparency, not a negative.