S4. DOVE S EXPERIMENTS ON THE 



the axes of the lamella. The particular construction of this natural po- 

 larizing apparatus described by Erraan, which from the thinness of the 

 lamella exhibits the systems of rings of an unusual size, and considerably 

 removed toward their optical axes on account of the obliquity of the 

 surface of emergence, is obtained optically by comparing these systems 

 of rings seen without previous polarization, in size and position, with 

 those which evolve light previously polarized rectilinearly and afterwards 

 analysed also around the optical axes of the including individuals, of 

 which the one serves for the polarizing, the other for the analysing ar- 

 rangement. That this last is the case, is moreover apparent from the 

 following observation, that when a tourmaline is revolved before the 

 crystal viewed in ordinary light, one of the systems of rings disappears 

 alternately without changing its form. As however the phaenomenon 

 remains the same when the crystal is revolved, the same holds good for 

 the polarizing prism, with which also the alterations of intensity of the 

 rings agree when the crystal is viewed with the naked eye in rectilinearly 

 polarized light. A decisive proof, however, that the individual behind 

 polarizes rectiUnearly, lies, as it seems to me, in the following fact, that 

 the rings seen with the naked eye do not take the form which corre- 

 sponds with the light when this is circularly incident. 



The third case, in which the axis of the lamina growing into the other 

 is inclined at an angle toward the axis of the including crystal, is also of 

 importance for uniaxal crystals. The modification of the system of rings 

 round the axis of the including ciystal thus produced must coincide 

 with that in two exactly central plates when a crystallized lamina of de- 

 finite thickness is inserted between them. As that lamina may here be 

 replaced by another similarly acting crystal, this case may be treated in 

 the same way without difficulty. Among seven plates of Iceland spar 

 exhibiting a deviation from the usual system of rings, I found two which 

 produced a very regular figure, namely, a black cross with curves alter- 

 nately osculating, which appeared to me to be circles and lemniscates ; 

 the interior curve was completely entwined into a figure of 8. If the 

 plate is turned in its own plane, the interior part of the system of rings 

 consists of four triangular vacant spaces. I obtained precisely the same 

 phaenomena by inserting a lamina of mica of definite thickness between 

 two plates exactly centred and producing the regular system of rings, 

 and by turning that lamina in its own plane. 



7. Experiments on Circular Polarization by other Modifications. 

 Fluor spar is the only crj'stallized substance of the regular system 

 which I have examined with respect to the effect cff an unequal distri- 

 bution of temperature within the body. The fragment I used in this in- 

 stance was quite colourless and transparent, 14^ inch long, and was lent to 

 me for these experiments by Professor Weiss. At a heat in which the 

 difference of path hatl become \ undulation in the glass cube, it exhi- 



