The optical disk is a device for demonstrating and making measurements on the phenomena of reflection and refraction. The first reference I have seen to it is in the Max Kohl catalogue, ca. 1900, where it is listed as Kolbe's apparatus to demonstrate the refraction of light.

   The apparatus below is listed as the "Kolbe-Harcourt Self-Contained Universal Inclination Optical Disk" in the 1916 catalogue of the L. E. Knott Apparatus Co. of Boston and cost $16.50. Harcourt was a captive brand of apparatus sold by Knott. The various attachments (converging and diverging lenses, prisms, refraction half-disks and water tanks) are contained in the sliding compartment in the base. At the top can be seen the series of slits that were used to send parallel rays across the various optical components. For $5.50 you could add a a series of narrow linear lenses that would produce a series of diverging rays, and a hollow, semi-circular refraction tank was $3.35. The attachment to produce and analyze polarized light cost $8.25. The apparatus could be used either for demonstration in the classroom or for experiments by individual pupils in the laboratory. This apparatus is in the collection at Dartmouth College.

At the right is a portion of a page from the 1900 Max Kohl catalogue showing how the optical disk was used. Diagrams on the previous page showed how a mirror could be used to confirm the law of specular reflection, and how a semi-circle of glass could be used to make measurements to demonstrate Snell's law.     In the top row, the demonstrations are total internal reflection, refraction through a parallel-sided slab of glass and refraction by a prism. The second row shows refraction in a prism of water, the focussing of parallel rays by a thick plano-convex lens and by focussing by a thinner positive lens. On the bottom there is the action of a negative lens on parallel rays, and the action of concave and convex mirrors on parallel rays.     Many of these experiments could also be done with the reflection and refraction apparatus made by Duboscq of Paris.

The Central Scientific Company and the W. M. Welch Scientific Company of Chicago made almost identical optical disks of the form developed by Prof. Hans Hartl (see below). Rays of light from a light source were rendered parallel by a lens system, passed through the series of horizontal slots shown at the right of the apparatus, and then passed through various lenses, triangular prisms and parallelepiped-shaped blocks. The face of the disk was marked off to show where to place the various reflectors and refractors. The apparatus at the right, from the Greenslade Collection, was made by Welch ($22.50 in the 1928 catalogue), but the refraction tank was made by Central Scientific. The parts were interchangeable.

Here is a group of reflectors and refractors made by several apparatus manufacturers; everyone made essentially the same accessories.    The unusual device in the picture is the air lens in the upper left-hand corner,     The half circle is used for studying Snell's Law.    The optical disk could be used to demonstrate polarization phenomena using a polariscope attachment.

Another form of optical disk was invented by B. Kolbe in 1900. It is described in the Max Kohl catalogue as follows:    "The apparatus consists essentially of a ground glass disc capable of rotation in rollers, provided with a suitable sine graduation [around the outer rim], and carrying a spring clip in order to easily interchange the light-refracting bodies. The following are given in with apparatus: 1 Screen with two diaphragms; 1 cardboard disk with marked degree graduations; 4 diaphragms with 1, 3, 7 and 9 gaps; 1 reflecting mirror; 1 solid half-cylinder of glass; 1 hollow half-cylinder of glass; 1 flint glass prism; 1 glass block; 1 cylindrical condensing lens and 1 cylindrical dispersion lens each 60 mm focus; 1 concave and 1 convex mirror each 100 mm radius of curvature; 1 glass body with two plane parallel surfaces and a refracting angle of 45 degrees and one of 60 degrees."    The apparatus is at Hobart and William Smith Colleges in Geneva, New York.

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