92 DOVE’S DESCRIPTION OF AN APPARATUS 
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 pre- 
cisely the same phenomena. 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 undark- 
ened room. 
2. The easy change of the linear into circular and elliptic polari- 
zation. 
3. Its rendering unnecessary a particular arrangement for illumi- 
nation. 
4. The extent of the field of view*. 
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,, s,, s, form the eye- 
glass, s, becomes the stand for the microscopic objects). 
7. The easy execution of all single changes in the various experi~ 
ments above described. 
The mechanician Hirschmann, of this place [ Berlin]; whose Nicol’s 
prisms are in the hands of many natural philosophers, has already exe- 
cuted 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 be are perpendicular | 
to the parallel surfaces ac and 4d; but, on the contrary, a6 is inclined at 
45° towards ad, and ed towards bd. The light therefore, falling per- _ 
pendicularly upon ad, will after being reflected by ab and ed proceed | 
from bd. The prismatic ares bounding the vacant space of total and 
partial reflexion, therefore, intersect each other, as in the annexed 
figure. In the vacate the light, after two reflexions, is un- | 
polarized ; in the vacant spac®s o and x it is polarized perpendicularly ; 
* Tn order not to diminish this, the arm Sf must move to and fro, close by wu. 4 
The cylindrical setting of the polarizing prism must not be higher than half an 
inch, 
