272 Transactions of the Boyal Microscopical Society. 
Table of the Wave-lengths in Millionths of a Millimeter corresponding 
to each ^^^th Division between the Various Bands, and of the Quantities 
to be deducted for each intermediate y^th of the Half Eevolution. 
Readings of 
tli6 Scale, 
Wave-lengths 
for eacli y'Qtti, 
Deduction for 
Readings of 
ttie Scale, 
Wave-lengths 
for each ^^th. 
Deduction for 
each Y^^th, 
•7 
687 
•8 
4-3 
488 
•3 
•8 
679 
•9 
4-4 
485 
•3 
•9 
670 
•9 
4-5 
482 
•3 
1-0 
661 
•8 
4-6 
479 
•3 
11 
653 
•8 
4-7 
476 
•3 
1-2 
645 
•8 
4-8 
473 
•3 
1-3 
637 
•7 
4-9 
470 
•2 
1-4 
630 
•6 
5-0 
468 
•4 
1-5 
624 
•7 
51 
464 
•3 
1-6 
617 
•8 
5-2 
461 
•2 
1-7 
609 
•6 
5-3 
459 
•3 
1-8 
603 
•6 
5-4 
456 
•3 
1-9 • 
597 
•8 
5-5 
453 
•2 
2-0 
589 
■5 
5-6 
451 
•2 
21 
584 
•6 
5-7 
449 
• 2 
2-2 
578 
•5 
5-8 
447 
•2 
2-3 
573 
•6 
5-9 
445 
•2 
2-4 
567 
•5 
6-0 
443 
•2 
2-5 
562 
•5 
6-1 
441 
•3 
2-6 
557 
•5 
6-2 
438 
•3 
2-7 
552 
•5 
6-3 
435 
•3 
2-8 
547 
•4 
6-4 
432 
•2 
2-9 
543 
•5 
6-5 
430 
•1 
3-0 
538 
•5 
6-6 
429 
■1 
31 
533 
■4 
6-7 
428 
•3 
3-2 
529 
•5 
6-8 
425 
•2 
o o 
524 
•4 
6'9 
423 
• X 
3-4 
520 
•3 
7-0 
422 
•3 
3-5 
517 
•4 
71 
419 
•1 
3-6 
513 
•5 
7-2 
418 
•1 
3-7 
508 
•3 
7-3 
417 
•1 
3-8 
505 
•3 
7-4 
416 
•3 
3-9 
502 
•4 
7-5 
413 
•1 
4-0 
498 
■3 
7-6 
412 
•2 
4-1 
495 
•2 
7-7 
410 
•2 
42 
493 
•5 
7-8 
408 
•1 
The chief objections against this apparatus are, that it is some- 
what difficult to obtain a suitable piece of quartz, and to cut it so 
that the light may pass exactly in the line of the principal axis of 
the crystal. Very often, though it may be perfectly clear and free 
from visible flaws, it may contain imperfections in the form of 
crystals arranged in a different optical position, easily seen with 
polarized light. Then again, though a tolerably well-formed crystal 
may be selected, much care is necessary to so cut and mount it that 
the two parallel faces are exactly perpendicular to the principal 
axis, along which there is no double refraction, but only true and 
perfect circular polarization. However, with care this can be done, 
