crystallized bodies on homogeneous light. 75 
This suggests a very simple and pleasing experiment, 
which affords an ocular demonstration of the adequacy of 
the explanation I have advanced. Let a plate of Rochelle 
salt be placed in a tourmaline apparatus in any azimuth (45 0 
is the most convenient) and firmly fixed on a proper stand 
in a dark room. The eye being now applied, let an assistant 
illuminate the emeried glass or lens of short focus* which 
disperses the light previous to its incidence on the first tour- 
maline, with the several colours of the prismatic spectrum in 
succession, beginning with the red. The rings will then be 
seen formed successively of each of the colours, perfectly 
regular in their figure, but contracting rapidly in dimension 
as they become illuminated with the more refrangible rays.-f* 
At the same time the pole about which they form will be seen 
to move regularly in the direction of the other axis of the 
crystal, and if we pass alternately from a red to a violet illu- 
mination, will shift its position accordingly, backwards and 
forwards through a very considerable angle. If rays of two 
colours be thrown at once on the apparatus, their two corres- 
ponding sets of rings will be seen at once, crossing, but not 
obliterating one another, and the distance between their re- 
spective centres will be observed to increase with the difference 
of their colours. 
By measuring the interval occupied by the projections of 
• See the description of an apparatus of this kind, subjoined. 
t See Lectiones Opticae, lib. ii. Pars. i. Obs. xiii. from which the idea of this 
experiment is taken. “ Magnaque voluptate perfusus” says Newton, with the 
enthusiasm of the true philosopher who loves the field he labours in, “ videbam 
eos dilatare aut contrahere se gradatim pro eo ac color luminis irrimutabatur.” 
It is impossible to witness the very beautiful phenomenon described in the text 
without entering into the same train of feeling. 
