Way cool! What I know (
courtesy Anonymous Works) about Herbert C. McKay, (1895-1971) is that he was an inventor and photographic enthusiast who, in 1924, authored the first book on amateur 16 mm cinematography. He was also the first person in the U.S to conduct a radio program on photography, in 1928. Recently discovered from his estate, McKay created a series of experimental multiple exposure photos of his wife, an example of which is featured in one of these stereo slides. She appears in the other as well, this time outdoors and facing the camera, in the same yellow dress, holding a flower the same very bright red as her lipstick in the other--making these, in my opinion, quite a compelling as well as highly cinematic feeling pair, with more than a bit of a Hitchcock-ian feel to me, a la Vertigo (and do making it just right that they should be preserved in double in viewers like these; doubles upon doubles upon doubles!
Much more about McKay here: https://archivesusie3d.wixsite.com/3-dlegends/herbert-c-mckay
Viewers measure 4" x 1 3/4" each. Transparencies are 1" tall and wide each. Excellent condition. Sold as a pair.