S4: DOVE’S EXPERIMENTS ON THE 
the axes of the lamella. The particular construction of this natural po- 
larizing apparatus described by Erman, 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 phenomenon 
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 rectilinearly, 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 crystal 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 
phznomena 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 crystallized substance of the regular system 
which I have examined with respect to the effect of an unequal distri- 
bution of temperature within the body. The fragment I used in this in- 
stance was quite colourless and transparent, 12 inch long, and was lent to 
me for these experiments by Professor Weiss. At a heat in which the 
difference of path had become § undulation in the glass cube, it exhi- 
te 
