The 35mm slide set contains approximately 1000 slides. The majority of these are illustrations and photographs from the textbook. The thought here was to provide instructors with images that they would be less likely to find or develop on their own. Most instructors will have access to a software program like PowerPoint if they want to develop their own text slides. The slide set on CD-ROM contains almost 4000 slides.
Flexibility is the key here as well.
If instructors want to develop 35mm slides from the CD-ROM, they have the option to do that.