The ornamental design for a viewer for slides, as shown and described.
1. Viewer for slides
1.1: Perspective
1.2: Front
1.3: Back
1.4: Left
1.5: Right
1.6: Top
1.7: Bottom
1.8: Reference
The design is the shape of the viewer for slides; the material of this design is synthetic resin or metal; the design shows a combination of unique shapes; reproduction 1.8forms no part of the claimed design.
The broken lines depict portions of the viewer for slides that form no part of the claimed design.