One of the most popular insulator styles, shown here in a drawing from McDougald's Price Guide, is called "the Mickey Mouse" after its exaggerated cable groove.

"Mickey Mouse" insulators were made by the Hemingray Glass Company for use in power lines carrying low voltages. The wire, or cable, that carried the electricity rested in the cable groove at the top of the insulator. The "ears" provided a means for tying the electrical wire to the insulator with a smaller strand of wire called a "tie wire".

Thirty-five different embossings have been found on CD 257's. Some of the older versions are engraved with two patent dates. The date of June 17, 1890. refers to the cable groove on top of the insulator, an invention by Samuel Oakman. The other patent date is May 2, 1893, which refers to the Hemingray patent for drip points.

Collector's fondly refer to a CD 257 insulator not only as a "Mickey Mouse", but also as a "Mickey", or "Mouse Ears", or sometimes just as a "Mouse".

Colors range from clear glass, aqua, emerald green, many aqua and bluish shades, a number of two-tone colorations, and a beautiful blue color collectors call "electric blue." 

Hemingray advertised the insulator in its catalogs as style #60, or 60-A. They were priced, in the 1920 catalog, at $140 per thousand. That's 14-cents apiece. Today, dark electric blue specimens are valued at $1,750 each.

References:
Insulators Price Guide, by John & Carol McDougald, Box 21157, Sedona, AZ 86341.
The Hemingray Glass Company, by H.G. "Bea" Hyve, 3269 N. Mountain View Dr, San Diego, CA 92116-1736