92 dove's description of an apparatus 



an interposed lamp. If instead of looking into tlie prism u we look 

 into e, on a slight change of the distance of the lens we obtain pre- 

 cisely the same phaenomena. 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 Sc^,, s^, s^ form the eye- 

 glass, Si 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-doUars. 



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 bd; but, on the contrary, ab is inclined at 

 45° towards ad, and cd towards bd. The light therefore, falling per- 

 pendicularly 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 vacan^jiace m, the light, after two reflexions, is un- 

 polarized ; in the vacant spa«^ o and n it is polarized perpendicularly ; 



• In order not to diminish this, the aim /must move to and fro, close by ti. 

 The cylindrical setthig of the polarizing prism must not be higher than half an 

 inch. 



