Page:Scientific Memoirs, Vol. 1 (1837).djvu/104

an interposed lamp. If instead of looking into the prism $u$ we look into $e$, on a slight change of the distance of the lens we obtain precisely the same phænomena. Thus an inverted order may also be given to all the stands with respect to the condensing-lens.

The superior advantages of the apparatus just described appear to me to be as follows:

1. The intensity of its light, which is so great that the flame of spirit of wine, 12 feet distant and coloured yellow by common salt, exhibits the system of rings of Iceland spar with great distinctness in an undarkened room.

2. The easy change of the linear into circular and elliptic polarization.

3. Its rendering unnecessary a particular arrangement for illumination.

4. The extent of the field of view^[1].

5. The purity of the colours, which are produced by colourless crystals only.

6. The cheapness of the instrument, since it serves equally as a model of an open telescope and as a microscope (the condensing-lens is the object-glass of the telescope; the stands $s_{2}$, $s_{3}$, $s_{5}$ form the eyeglass, $s_{4}$ becomes the stand for the microscopic objects).

7. The easy execution of all single changes in the various experiments above described.

The mechanician Hirschmann, of this place [Berlin], whose Nicol's prisms are in the hands of many natural philosophers, has already executed this apparatus according to my instructions in several sets made to order. Its price, if it is to be used both as an open telescope and microscope, is 60 rix-dollars.

Postscript.

Fig. 4. Plate II. represents a small apparatus consisting of a single piece of glass, which exhibits united the modifications of the light by reflexion. The mutually parallel surfaces ad and bc are perpendicular to the parallel surfaces ac and bd; but, on the contrary, ab is inclined at 45° towards ad, and cd towards bd. The light therefore, falling perpendicularly upon ad, will after being reflected by ab and cd proceed from bd. The prismatic arcs bounding the vacant space of total and partial reflexion, therefore, intersect each other, as in the annexed figure. In the vacant space m, the light, after two reflexions, is un-polarized; in the vacant spaces o and n it is polarized perpendicularly;

1. In order not to diminish this, the arm $f$ must move to and fro, close by $u$. The cylindrical setting of the polarizing prism must not be higher than half an inch.