INTRODUCTION:
This Prism Rotator is an accessory which fits on a microscope stage, where it enables viewing of all sides of an object. Ideally, the object to be studied is about 3-5 mm in size. It is placed atop a central prism, within a round, glass-bordered, rotatable chamber. This prism enables views of the top and bottom of the object. A second prism is fixed, suspended from above the periphery of the chamber, and aimed to receive light from the side of the object, so that when the chamber is manually rotated, all sides of the object may be viewed. There are 3 hooked, metal arms around the periphery of the chamber, from which small electric lamps were to be suspended.

DESCRIPTION:

This is a very well made device made of a metal alloy, likely German Silver, and is signed on one end in fancy script, Carl Zeiss Jena. It comes in its original box embossed PRISMEN-ROTATOR in the center of the lid and C.ZEISS JENA at the right lower front corner of the lid. The box measures 133 X 74.5 X 66mm high in maximal dimensions. The box is lined with blue velvet. The instrument has a fixed disk of 35 mm diameter on its bottom concentric with the rotating chamber. The black area seen on the bottom of the instrument is slightly recessed, so that the disk, which is otherwise even with the bottom ends of the instrument, fits into the metal ring at the bottom of the case. Presumably this part of design allows the device to be centered in a round microscope stage with a suitable outer diameter about 75 mm with a 35 mm central opening. The case has a fancy metal catch.

The device measures 108 mm X 41.5 mm. It has a circular metal plate that forms the base of a glass-enclosed chamber which allows objects to be studied in liquid media. The inside diameter of this chamber is about 36 mm. Attached to the center of the bottom of the chamber is a rectangular block containing a V-shaped double-reflecting prism. This block measures 7 X 10 mm, and rotates with the chamber. The reflecting surface of the prism allows examination of the bottom of an object placed on one half of it via double reflection upward. The top of the object can simply be viewed from above. The light paths are depicted in the figure to the left.

Around the base of the chamber, outside the glass, is an inclined metal scale with markings every degree and labeled in tens of degrees (0-35). The outer diameter of this part is 52 mm and thus extends beyond the edges of the top plate of the device.

A second prism box measuring 15 X 7 mm, is fixed in position, suspended from above the chamber by an L-shaped bracket attached to a post. As shown in the figure to the left, this prism is constructed with reflecting surfaces to direct the image from the side of the object being studied upward so the side of the object can also be studied from above. Rotating the chamber, with the object sitting on the central prism, allows all sides of the object to be seen without disturbing the object.

The round plate at the bottom of the chamber sits on top of a plate that is dovetailed to the plate underneath it so that it can be moved right or left about a cm, with a stop to register it in the center of the device. Under this plate is another plate dovetailed to the bottom plate of the device that allows it to move forward up to about 7 or 8 mm, or back to its original position.

Adjustible supports hold hooks inside rubber gromets from the three upright supports. These hooks were provided to support up to three tiny electric light bulbs to illuminate the object from 3 points.

USAGE:
The object to be studied is placed on top of the inward half part of the central prism box that rotates, centered on that half of the prism. The top of the object is observed in the usual manner from above. To observe the bottom of the object, the microscope objective is positioned over the outer part of the central prism so that the double reflecting surface of this prism provides a view of the bottom of the object being studied. To observe the sides of the object, the objective is positioned over the upward reflecting surface of the fixed prism; with the objective in this position, rotating the chamber holding the center prism allows the observer to visualize all sides of the object being studied. According to Zeiss catalogs, the apparatus was used for observing delicate objects between 3 mm and 5 mm in size, thus a narrow size-range of objects could be examined with this device. This is in agreement with the area of the central prism that the object would be placed on. Insect eggs were given as an example of a typical subject for which this device was ideal. The illustrations provided above show the head of an insect to illustrate the effects. A long working distance low power objective works best; a stereo dissecting microscope is ideal. The posts interfere with work using a multi-objective nosepiece, objectives which have a wide outer diameter, or objectives with short working distances. I took my images using one port of a B & L dissecting stereomicroscope which provides a nice long working distance. The movie demonstrates how the rotational movement allows one to observe every side of the subject without disturbing it. When sold, lighting was provided by small electric light bulbs suspended from the hooks. Today it is simpler to just use a high intensity light beam from the side or above. One must note that, the microscope must be refocused when changing from top view to the reflected bottom view. As illustrated in the movie, as the object rotates, if it is not perfectly centered on the central prism, and radially symmetric, periodic refocusing will be needed.

HISTORY:

The prism rotator was invented by Greenough in the last decade of the 19th century. It was first manufactured by Carl Zeiss in 1897. It was reported in Zeitschr. fur wiss. Mikroskopie Vol XIV, p 304 in November of 1898. It was often advertised and it was offered intermittently in Zeiss catalogs from about 1898 to at least 1934 or later.

In earlier versions, light could not only be directed from above or from the sides, but also from below. As shown in the image of the light path to the left, light could pass through a partially silvered surface of the prism, allowing the light to pass through, illuminating the bottom of the object, and then reflecting the light off the object back down and then, to the opposite side of the prism, and from there, upward. An engraving depicting this earlier Prism Rotator is shown to the right. Although this arrangement was apparently abandoned early on, prior to 1898, the diagram illustrating one of the light paths from below continued to be used along side the newer models from 1898 until 1930, when it was finally changed to omit that light path. I know of no surviving instruments with the older design, allowing illumination from beneath the stage, in public or private collections.

Besides the changes noted above, the prism rotator was offered in two forms, one with a single fixed side prism combination and a second with two fixed side prism combinations to permit correct orientation of the image so that the image is not reversed, allowing faithful right-left orientation during photography or drawing.

As shown here to the left, the type of fixed prism was changed in the 1930s.

The catalog illustrations, and all the other extant examples I have seen have two light support hooks, though the example featured at the top of this page has three.

Although offered for many years, the Zeiss Prism Rotator is not commonly offered for sale these days (in the 2020s), and when it is, it always commands a high price at auction.